Fair Value Measurements and Derivative Instruments (Offsetting of Derivative Instruments) (Details) - USD ($) $ in Thousands	Dec. 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	$ 65,297	$ 320,385
Gross Amount of Eligible Offsetting Recognized Derivative Liabilities	(60,303)	(104,751)
Cash Collateral Received	0	0
Net Amount of Derivative Assets	4,994	215,634
Offsetting of Financial Liabilities under Master Netting Agreements [Abstract]
Gross Amount of Derivative Liabilities Presented in the Consolidated Balance Sheet	(201,812)	(115,961)
Gross Amount of Eligible Offsetting Recognized Derivative Assets	60,303	104,751
Cash Collateral Pledged	0	0
Net Amount of Derivative Liabilities	$ (141,509)	$ (11,210)