Swap Agreements by Counterparty (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional Amount	$ 1,836,000	$ 1,771,000
Chicago Mercantile Exchange
Derivative [Line Items]
Notional Amount	1,026,000	786,000
JPMorgan Securities
Derivative [Line Items]
Notional Amount	350,000	425,000
Deutsche Bank Securities
Derivative [Line Items]
Notional Amount	225,000	325,000
RBS Greenwich Capital
Derivative [Line Items]
Notional Amount	115,000	115,000
Nomura Securities International
Derivative [Line Items]
Notional Amount	100,000	100,000
Bank of New York
Derivative [Line Items]
Notional Amount	20,000	20,000
Interest rate swap agreements
Derivative [Line Items]
Notional Amount	$ 1,836,000	$ 1,771,000