Fair Value Measurements and Derivative Instruments (Offsetting of Derivative Instruments) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Offsetting of Financial Liabilities under Master Netting Agreements [Abstract]
Gross Amount of Derivative Liabilities Presented in the Consolidated Balance Sheet	$ (1,044,292)	$ (767,635)
Gross Amount of Eligible Offsetting Recognized Derivative Assets	129,815	63,981
Net Amount of Derivative Liabilities	(914,477)	(703,654)
Offsetting of Financial Assets under Master Netting Agreements [Abstract]
Gross Amount of Derivative Assets Presented in the Consolidated Balance Sheet	134,574	63,981
Gross Amount of Eligible Offsetting Recognized Derivative Liabilities	(129,815)	(63,981)
Net Amount of Derivative Assets	$ 4,759	$ 0