Fair Value Measurements - Derivative Instrument Assets and Liabilities Measured at Fair Value (Details) - Fair value on a recurring basis - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Quoted prices in active markets for identical assets (Level 1)
Assets
Derivative instruments, at fair value	$ 0	$ 0
Liabilities
Derivative instruments, at fair value (included in other current liabilities)	0	132
Significant other observable inputs (Level 2)
Assets
Derivative instruments, at fair value	395,264	458,460
Liabilities
Derivative instruments, at fair value (included in other current liabilities)	26,057	22,810
Significant unobservable inputs (Level 3)
Assets
Derivative instruments, at fair value	0	0
Liabilities
Derivative instruments, at fair value (included in other current liabilities)	0	0
Fair value
Assets
Derivative instruments, at fair value	395,264	458,460
Liabilities
Derivative instruments, at fair value (included in other current liabilities)	$ 26,057	$ 22,942