Fair Value Measurements and Derivative Instruments - Offsetting of Derivative Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Offsetting of Financial Assets under Master Netting Agreements [Abstract]
Gross Amount of Derivative Assets Presented in the Consolidated Balance Sheet	$ 30,883	$ 65,297
Offsetting of Financial Liabilities under Master Netting Agreements [Abstract]
Gross Amount of Derivative Assets Presented in the Consolidated Balance Sheet	(385,576)	(201,812)
External Counterparties
Offsetting of Financial Assets under Master Netting Agreements [Abstract]
Gross Amount of Derivative Assets Presented in the Consolidated Balance Sheet	30,883	65,297
Gross Amount of Eligible Offsetting Recognized Derivative Liabilities	(30,067)	(60,303)
Cash Collateral Received	0	0
Net Amount of Derivative Assets	816	4,994
Offsetting of Financial Liabilities under Master Netting Agreements [Abstract]
Gross Amount of Derivative Assets Presented in the Consolidated Balance Sheet	(385,576)	(201,812)
Gross Amount of Eligible Offsetting Recognized Derivative Assets	30,067	60,303
Cash Collateral Pledged	0	0
Net Amount of Derivative Liabilities	$ (355,509)	$ (141,509)