Derivative Instruments - Effect on Consolidated Results (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gains (losses) on derivatives	$ 0	$ (132,171)	$ 201,000	$ (221,486)
Net gains (losses) on derivatives	0	(237,079)	(304,052)	(441,717)
Realized gains (losses) on terminations of interest rate swaps	0	95,211	463,966	203,257
Interest Rate Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gains (losses) on derivatives	0	(130,639)	204,611	(224,714)
Net realized gains (losses) on derivatives	0	(91,288)	(470,352)	(193,873)
Treasury Futures
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gains (losses) on derivatives	0	(1,197)	(3,611)	4,128
Net realized gains (losses) on derivatives	0	(13,544)	$ (34,700)	(26,123)
Swaptions
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gains (losses) on derivatives	0	(335)		(900)
Net realized gains (losses) on derivatives	$ 0	$ (76)		$ (235)