Fair Value Measurements and Derivative Instruments (Offsetting of Derivative Instruments) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Offsetting of Financial Assets under Master Netting Agreements [Abstract]
Gross Amount of Derivative Assets Presented in the Consolidated Balance Sheet	$ 19,397	$ 134,574
Gross Amount of Eligible Offsetting Recognized Derivative Liabilities	(19,397)	(129,815)
Net Amount of Derivative Assets	0	4,759
Offsetting of Financial Liabilities under Master Netting Agreements [Abstract]
Gross Amount of Derivative Liabilities Presented in the Consolidated Balance Sheet	(373,497)	(1,044,292)
Gross Amount of Eligible Offsetting Recognized Derivative Assets	19,397	129,815
Cash Collateral Pledged	7,213
Net Amount of Derivative Liabilities	$ (346,887)	$ (914,477)