Fair Value Measurements and Derivative Instruments - Offsetting of Derivative Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Offsetting of Financial Assets under Master Netting Agreements [Abstract]
Gross Amount of Derivative Assets Presented in the Consolidated Balance Sheet	$ 391,116	$ 320,385
Gross Amount of Eligible Offsetting Recognized Derivative Liabilities	(110,989)	(104,751)
Cash Collateral Received	0	0
Net Amount of Derivative Assets	280,127	215,634
Offsetting of Financial Liabilities under Master Netting Agreements [Abstract]
Gross Amount of Derivative Liabilities Presented in the Consolidated Balance Sheet	(118,117)	(115,961)
Gross Amount of Eligible Offsetting Recognized Derivative Assets	110,989	104,751
Cash Collateral Pledged	0	0
Net Amount of Derivative Liabilities	$ (7,128)	$ (11,210)