Fair Value Measurement and Derivative Instruments - Offsetting of Derivative Instruments (Details) - USD ($) $ in Thousands	Jun. 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	$ 118,046	$ 63,981
Gross Amount of Eligible Offsetting Recognized Derivative Liabilities	(116,059)	(63,981)
Net Amount of Derivative Assets	1,987	0
Offsetting of Financial Liabilities under Master Netting Agreements [Abstract]
Gross Amount of Derivative Liabilities Presented in the Consolidated Balance Sheet	(726,610)	(767,635)
Gross Amount of Eligible Offsetting Recognized Derivative Assets	116,059	63,981
Net Amount of Derivative Liabilities	$ (610,551)	$ (703,654)