Debt - Schedule of Outstanding Debt Obligations (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term Debt	$ 742,535	$ 734,683
Less: current maturities	20,727	0
Long-term debt, net of current maturities	721,808	734,683
Senior Notes \| 2023 Senior Notes
Debt Instrument [Line Items]
Long-term Debt	466,906	466,291
Convertible Debt
Debt Instrument [Line Items]
Long-term Debt	275,629
Convertible Debt \| 2022 Convertible Notes
Debt Instrument [Line Items]
Long-term Debt	254,902	248,194
Convertible Debt \| 2019 Convertible Notes
Debt Instrument [Line Items]
Long-term Debt	$ 20,727	$ 20,198

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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