Derivative Instruments - Notional and Fair Value Amounts of Derivative Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Amount	$ 307,500	$ 278,200
Fair Value	10,685	2,647
Bank of America, N.A.
Derivative [Line Items]
Notional Amount	145,100	144,500
Fair Value	5,026	870
Barclays Bank PLC
Derivative [Line Items]
Notional Amount	101,700	28,500
Fair Value	2,271	247
Citigroup Inc.
Derivative [Line Items]
Notional Amount	0	0
Fair Value	0	0
Credit Suisse International
Derivative [Line Items]
Notional Amount	0	16,100
Fair Value	0	55
Societe Generale
Derivative [Line Items]
Notional Amount	60,700	89,100
Fair Value	$ 3,388	$ 1,475