Fair Value Measurements and Derivative Instruments - Offsetting of Derivative Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Offsetting of Financial Assets under Master Netting Agreements [Abstract]
Gross Amount of Derivative Assets Presented in the Consolidated Balance Sheet	$ 113,279	$ 63,981
Gross Amount of Eligible Offsetting Recognized Derivative Liabilities	(112,567)	(63,981)
Net Amount of Derivative Assets	712	0
Offsetting of Financial Liabilities under Master Netting Agreements [Abstract]
Gross Amount of Derivative Liabilities Presented in the Consolidated Balance Sheet	(925,202)	(767,635)
Gross Amount of Eligible Offsetting Recognized Derivative Assets	112,567	63,981
Net Amount of Derivative Liabilities	$ (812,635)	$ (703,654)