Derivative Instruments - Swap Agreements by Counterparty (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional Amount	$ 3,146,000	$ 2,681,000
Chicago Mercantile Exchange
Derivative [Line Items]
Notional Amount	3,146,000	2,421,000
JPMorgan Securities
Derivative [Line Items]
Notional Amount		50,000
Deutsche Bank Securities
Derivative [Line Items]
Notional Amount		75,000
RBS Greenwich Capital
Derivative [Line Items]
Notional Amount		115,000
Bank of New York
Derivative [Line Items]
Notional Amount		20,000
Interest rate swap agreements
Derivative [Line Items]
Notional Amount	$ 3,146,000	$ 2,681,000