Derivative Instruments - Effect on Consolidated Financial Condition (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Notional Amount Outstanding	$ 4,731,000	$ 11,579,400
Derivatives, at fair value, net	3,611	37,468
Derivative Liabilities, Net Estimated Fair Value/Carrying Value	0	0
Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Notional Amount Outstanding	4,111,300	10,906,700
Derivatives, at fair value, net	0	35,798
Derivative Liabilities, Net Estimated Fair Value/Carrying Value	0	0
Swaptions
Derivatives, Fair Value [Line Items]
Notional Amount Outstanding	0	53,000
Derivatives, at fair value, net	0	1,670
Derivative Liabilities, Net Estimated Fair Value/Carrying Value	0	0
Treasury Futures
Derivatives, Fair Value [Line Items]
Notional Amount Outstanding	619,700	619,700
Derivatives, at fair value, net	3,611	0
Derivative Liabilities, Net Estimated Fair Value/Carrying Value	$ 0	$ 0