Derivative Instruments - Additional Information (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
5-year U.S. Treasury Note Futures
Derivative [Line Items]
Notional Amount	$ 0	$ 21,600,000
Maturity date		2018-03
10-year U.S. Treasury Note Futures
Derivative [Line Items]
Notional Amount	$ 700,000,000	$ 650,000,000	$ 700,000,000	$ 350,000,000	$ 350,000,000	$ 0
Maturity date	2018-12	2018-03
Derivative outstanding options	$ 0	$ 0