Derivative Instruments - Notional and Fair Value Amounts of Derivative Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional Amount	$ 278,200	$ 246,800
Fair Value	2,647	15,550
Bank of America, N.A.
Derivative [Line Items]
Notional Amount	144,500	85,100
Fair Value	870	6,320
Barclays Bank PLC
Derivative [Line Items]
Notional Amount	28,500	48,900
Fair Value	247	1,828
Citigroup Inc.
Derivative [Line Items]
Notional Amount	0	0
Fair Value	0	0
Credit Suisse International
Derivative [Line Items]
Notional Amount	16,100	21,100
Fair Value	55	1,444
Societe Generale
Derivative [Line Items]
Notional Amount	89,100	91,700
Fair Value	$ 1,475	$ 5,958