Derivative Instruments - Effect on Consolidated Financial Condition (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Notional Amount Outstanding	$ 4,827,100,000	$ 2,640,600,000
Derivative Assets, Net Estimated Fair Value/Carrying Value	48,914,000	9,677,000
Derivative Liabilities, Net Estimated Fair Value/Carrying Value	(320,000)	(2,350,000)
Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Notional Amount Outstanding	3,816,400,000	1,396,900,000
Derivative Assets, Net Estimated Fair Value/Carrying Value	43,257,000	0
Derivative Liabilities, Net Estimated Fair Value/Carrying Value	0	0
Swaptions
Derivatives, Fair Value [Line Items]
Notional Amount Outstanding	391,000,000	624,000,000
Derivative Assets, Net Estimated Fair Value/Carrying Value	2,105,000	7,892,000
Derivative Liabilities, Net Estimated Fair Value/Carrying Value	(320,000)	(2,350,000)
Treasury Futures
Derivatives, Fair Value [Line Items]
Notional Amount Outstanding	619,700,000	619,700,000
Derivative Assets, Net Estimated Fair Value/Carrying Value	3,552,000	1,785,000
Derivative Liabilities, Net Estimated Fair Value/Carrying Value	$ 0	$ 0