Derivative Instruments - Notional and Fair Value Amounts of Derivative Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Amount	$ 300,400	$ 278,200
Fair Value	8,753	2,647
Bank of America, N.A.
Derivative [Line Items]
Notional Amount	156,800	144,500
Fair Value	3,560	870
Barclays Bank PLC
Derivative [Line Items]
Notional Amount	72,500	28,500
Fair Value	1,696	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	295	55
Societe Generale
Derivative [Line Items]
Notional Amount	55,000	89,100
Fair Value	$ 3,202	$ 1,475