Derivative Instruments - Effect on Consolidated Financial Condition (Detail) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Notional Amount Outstanding	$ 8,365,100,000	$ 4,827,100,000
Derivatives, at fair value, net	129,341,000	48,914,000
Derivative Liabilities, Net Estimated Fair Value/Carrying Value	0	(320,000)
Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Notional Amount Outstanding	7,692,400,000	3,816,400,000
Derivatives, at fair value, net	123,975,000	43,257,000
Derivative Liabilities, Net Estimated Fair Value/Carrying Value	0	0
Swaptions
Derivatives, Fair Value [Line Items]
Notional Amount Outstanding	53,000,000	391,000,000
Derivatives, at fair value, net	2,417,000	2,105,000
Derivative Liabilities, Net Estimated Fair Value/Carrying Value	0	(320,000)
Treasury Futures
Derivatives, Fair Value [Line Items]
Notional Amount Outstanding	619,700,000	619,700,000
Derivatives, at fair value, net	2,949,000	3,552,000
Derivative Liabilities, Net Estimated Fair Value/Carrying Value	$ 0	$ 0