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Senior, Convertible Senior Notes (Details Textual) (USD $)
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6 Months Ended | 12 Months Ended | 6 Months Ended | 12 Months Ended | 6 Months Ended | 12 Months Ended | 6 Months Ended | 12 Months Ended | 6 Months Ended | ||||||||
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Jun. 30, 2014
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Mar. 31, 2014
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Dec. 31, 2013
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Jun. 30, 2014
4.50% senior notes due two thousand eighteen [Member]
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Dec. 31, 2013
4.50% senior notes due two thousand eighteen [Member]
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Jun. 30, 2014
7.00% senior notes due 2022 [Member]
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Dec. 31, 2013
7.00% senior notes due 2022 [Member]
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Jun. 30, 2014
7.15% senior notes due 2020 [Member]
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Dec. 31, 2013
7.15% senior notes due 2020 [Member]
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Jun. 30, 2014
1.875% convertible senior notes due 2032 [Member]
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Dec. 31, 2013
1.875% convertible senior notes due 2032 [Member]
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Dec. 31, 2012
1.875% convertible senior notes due 2032 [Member]
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Jun. 30, 2014
Revolving Credit Facility [Member]
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Dec. 31, 2013
Revolving Credit Facility [Member]
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Jun. 30, 2014
Credit Facility Accordion Feature [Member]
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Jun. 30, 2014
Consolidated EBITDA [Member]
Revolving Credit Facility [Member]
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Jun. 30, 2014
Consolidated Interest Incurred [Member]
Revolving Credit Facility [Member]
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| Debt Instrument [Line Items] | |||||||||||||||||
| Line of credit facility, maximum borrowing capacity | $ 400,000,000 | $ 200,000,000 | $ 100,000,000 | ||||||||||||||
| Debt Instrument Maturity Year | 2018 | ||||||||||||||||
| Available For Issuance | 200,000,000 | ||||||||||||||||
| Minimum net worth required for compliance | 670,300,000 | ||||||||||||||||
| Leverage ratio | 60.00% | ||||||||||||||||
| Interest coverage ratio | 1.50 | 1.00 | |||||||||||||||
| Line of credit facility, amount outstanding | 0 | 0 | 0 | 0 | |||||||||||||
| Letters of Credit Outstanding, Amount | 26,743,000 | 42,068,000 | |||||||||||||||
| Line of Credit Facility, Remaining Borrowing Capacity | 373,300,000 | ||||||||||||||||
| Debt instrument, stated rate | 4.50% | 4.50% | 7.00% | 7.00% | 7.15% | 7.15% | 1.875% | 1.875% | |||||||||
| Percentage of wholly owned subsidiary | 100.00% | ||||||||||||||||
| Year in which senior notes due | 2018 | 2018 | 2022 | 2022 | 2020 | 2020 | 2032 | 2032 | |||||||||
| Aggregate principal amount of offering | 1,000 | ||||||||||||||||
| Debt Instrument, Unamortized Premium | $ 3,271,000 | $ 3,555,000 | |||||||||||||||
| Debt Instrument, Convertible, Conversion Ratio | 17.1985 | ||||||||||||||||
| Debt Instrument, Convertible, Conversion Price | $ 58.14 | ||||||||||||||||