Derivative Instruments - Offsetting of Derivative Assets and Liabilities in the Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivative assets
Gross amounts of recognized assets	$ 18,824	$ 60,105
Gross amounts offset in the statement of financial position	(4,578)	(2,440)
Net amounts of assets presented in the statement of financial position	14,246	57,665
Gross amounts of financial instruments not offset in the statement of financial position	0	0
Net amount	14,246	57,665
Derivative liabilities
Gross amounts of recognized liabilities	5,883	2,440
Gross amounts offset in the statement of financial position	(4,578)	(2,440)
Net amounts of liabilities presented in the statement of financial position	1,305	0
Gross amounts of financial instruments not offset in the statement of financial position	0	0
Net amount	$ 1,305	$ 0