Financial Instruments Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 60,475	$ 72,574	$ 176,985	$ 327,420
Long-term Debt	2,724,672	2,705,718
Quoted Prices in Active Markets for Identical Liabilities (Level 1)
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Derivative Asset	0	0
Guarantees, Fair Value Disclosure	0	0
Significant Other Observable Inputs (Level 2)
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Derivative Asset	(188,156)	266,558
Guarantees, Fair Value Disclosure	0	0
Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Derivative Asset	0	0
Guarantees, Fair Value Disclosure	$ (1,362)	$ (1,228)