Long-Term Debt - Senior Unsecured Notes (Details) - USD ($) $ in Thousands			9 Months Ended		12 Months Ended
	Feb. 29, 2016	Feb. 23, 2015	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Apr. 01, 2014
Senior Unsecured Notes
Repayment under long-term debt			$ 129,000	$ 42,000
Gain on debt extinguishment				$ 99,530
Senior notes
Senior Unsecured Notes
Aggregate principal amount repurchased					$ 190,900
Gain on debt extinguishment					99,500
2022 Notes | Senior notes
Senior Unsecured Notes
Aggregate principal amount						$ 500,000
Stated interest rate (as a percent)						6.75%
Aggregate principal amount repurchased					90,900
Purchased amount					38,100
Aggregate principle amount repurchased and available for reissuance					20,300
2023 Notes | Senior notes
Senior Unsecured Notes
Aggregate principal amount		$ 250,000
Stated interest rate (as a percent)		9.25%
Discount of principal amount (as a percent)		94.59%
Notes proceeds from issuance		$ 236,500
Beneficial owner | 2023 Notes | Senior notes
Senior Unsecured Notes
Aggregate principal amount repurchased	$ 100,000				100,000
Purchased amount					$ 46,500
Gain on debt extinguishment	$ 48,300