Derivative Instruments - Effect on Consolidated Results (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gains (losses) on derivatives									$ (106,209,000)	$ (141,162,000)	$ 47,976,000
Net gains (losses) on derivatives	$ 78,030,000	$ (136,671,000)	$ (237,079,000)	$ (204,638,000)	$ (315,877,000)	$ 74,078,000	$ 24,502,000	$ 94,504,000	(500,358,000)	(122,793,000)	6,188,000
Realized gains (losses) on terminations of interest rate swaps									(359,726,000)	0	(16,143,000)
Interest Rate Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gains (losses) on derivatives									(122,272,000)	(125,595,000)	43,697,000
Net realized gains (losses) on derivatives									(356,713,000)	(1,488,000)	(31,593,000)
Treasury Futures
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gains (losses) on derivatives									16,986,000	(16,928,000)	1,768,000
Net realized gains (losses) on derivatives									(37,032,000)	21,333,000	(4,061,000)
Swaptions
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gains (losses) on derivatives									(923,000)	1,361,000	2,511,000
Net realized gains (losses) on derivatives									$ (404,000)	$ (1,476,000)	$ (6,134,000)