Derivative Instruments - Effect on Consolidated Financial Condition (Detail) - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Notional Amount Outstanding	$ 2,637,600,000	$ 2,640,600,000
Net Estimated Fair Value/Carrying Value	10,889,000	9,677,000
Net Estimated Fair Value/Carrying Value	(1,627,000)	(2,350,000)
Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Notional Amount Outstanding	1,535,900,000	1,396,900,000
Net Estimated Fair Value/Carrying Value	5,876,000	0
Net Estimated Fair Value/Carrying Value	0	0
Swaptions
Derivatives, Fair Value [Line Items]
Notional Amount Outstanding	482,000,000	624,000,000
Net Estimated Fair Value/Carrying Value	5,013,000	7,892,000
Net Estimated Fair Value/Carrying Value	(1,627,000)	(2,350,000)
Treasury Futures
Derivatives, Fair Value [Line Items]
Notional Amount Outstanding	619,700,000	619,700,000
Net Estimated Fair Value/Carrying Value	0	1,785,000
Net Estimated Fair Value/Carrying Value	$ 0	$ 0