Derivative Instruments - Notional and Fair Value Amounts of Derivative Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Amount	$ 291,700	$ 278,200
Fair Value	7,643	2,647
Bank of America, N.A.
Derivative [Line Items]
Notional Amount	164,600	144,500
Fair Value	3,302	870
Barclays Bank PLC
Derivative [Line Items]
Notional Amount	32,600	28,500
Fair Value	808	247
Citigroup Inc.
Derivative [Line Items]
Notional Amount	0	0
Fair Value	0	0
Credit Suisse International
Derivative [Line Items]
Notional Amount	16,100	16,100
Fair Value	207	55
Societe Generale
Derivative [Line Items]
Notional Amount	78,400	89,100
Fair Value	$ 3,326	$ 1,475