Derivative Instruments and Hedging Activities - Effect of Derivative on Income Statement (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Unrealized gain (loss) on derivative instruments	$ 16,926	$ (38,950)	$ 56,686
Gain (loss) in derivative instruments not designated as hedges	16,926	(38,950)	56,686
Derivatives Not Designated as Hedges
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Unrealized gain (loss) on derivative instruments	8,870	(37,268)	(137,603)
Gain (loss) on derivative instruments | Derivatives Not Designated as Hedges
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Realized gain (loss) on derivative instruments	$ 8,056	$ (1,682)	$ 194,289