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Long-Term Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2016
Debt Instrument [Line Items]
Total long-term debt	$ 2,757,481	$ 2,097,606
Line of Credit [Member] \| Revolving Credit Facility [Member] \| Senior Secured Revolving Line of Credit [Member]
Debt Instrument [Line Items]
Total principal of senior unsecured notes and revolving line of credit	651,000	70,000
Line of Credit [Member] \| Revolving Credit Facility [Member] \| OMP Revolving Line of Credit [Member]
Debt Instrument [Line Items]
Total principal of senior unsecured notes and revolving line of credit	165,000	78,000
Senior Notes [Member]
Debt Instrument [Line Items]
Total principal of senior unsecured notes and revolving line of credit	2,039,409	2,052,950
Less: unamortized deferred financing costs on senior unsecured notes	$ (22,889)	(22,956)
Senior Notes [Member] \| 7.25% senior unsecured notes due February 1, 2019 [Member]
Debt Instrument [Line Items]
Stated interest rate (as percent)	7.25%
Total principal of senior unsecured notes and revolving line of credit	$ 0	54,275
Senior Notes [Member] \| 6.50% senior unsecured notes due November 1, 2021 [Member]
Debt Instrument [Line Items]
Stated interest rate (as percent)	6.50%
Total principal of senior unsecured notes and revolving line of credit	$ 71,835	395,501
Senior Notes [Member] \| 6.875% senior unsecured notes due March 15, 2022 [Member]
Debt Instrument [Line Items]
Stated interest rate (as percent)	6.875%
Total principal of senior unsecured notes and revolving line of credit	$ 901,480	937,080
Senior Notes [Member] \| 6.875% senior unsecured notes due January 15, 2023 [Member]
Debt Instrument [Line Items]
Stated interest rate (as percent)	6.875%
Total principal of senior unsecured notes and revolving line of credit	$ 366,094	366,094
Senior Notes [Member] \| 6.25% senior unsecured notes due May 1, 2026 [Member]
Debt Instrument [Line Items]
Stated interest rate (as percent)	6.25%
Total principal of senior unsecured notes and revolving line of credit	$ 400,000	0
Senior Notes [Member] \| 2.625% senior convertible notes due September 15, 2023 [Member]
Debt Instrument [Line Items]
Stated interest rate (as percent)	2.625%
Total principal of senior unsecured notes and revolving line of credit	$ 300,000	300,000
Less: unamortized debt discount on senior unsecured convertible notes	$ (75,039)	$ (80,388)	$ (93,200)