Derivative Instruments - Additional Information (Detail) - USD ($) $ in Thousands	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 [Line Items]
Gain (loss) on derivatives	$ 20,824	$ (24,734)	$ (53,543)	$ (27,289)	$ (44,504)	$ 13,090	$ 9,930	$ 13,412	$ (84,741)	$ (8,071)	$ 7,132
Interest rate swap agreements
Derivative [Line Items]
Derivative contract, weighted average maturity (in months)									48 months
Derivative Notional Value	$ 2,501,000				3,306,000				$ 2,501,000	3,306,000
Interest rate swap agreements | Minimum
Derivative [Line Items]
Fixed interest rate	1.411%								1.411%
Interest rate swap agreements | Maximum
Derivative [Line Items]
Fixed interest rate	3.2205%								3.2205%
TBA Agency MBS
Derivative [Line Items]
Derivative Notional Value	$ 250,000				$ 900,000				$ 250,000	900,000
Gain (loss) on derivatives									$ 14,166	$ (8,656)	$ (1,440)
Financial instrument, fixed-rate period									15 years
TBA Agency MBS | Minimum
Derivative [Line Items]
Fixed interest rate	3.00%								3.00%
TBA Agency MBS | Maximum
Derivative [Line Items]
Fixed interest rate	4.50%								4.50%