Derivative Instruments - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) item	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) derivative
Derivative [Line Items]
Aggregate notional amount	$ 2,281,000		$ 2,281,000		$ 1,771,000
AOCI relating to interest rate swaps			16,000
Interest rate cash flow hedge gain (loss) to be reclassified during next 12 months	3,500		3,500
Gain (loss) on interest rate swaps, net	1,530	$ 8,141	(3,957)	$ (58,167)
Gain on derivatives	(945)	9,493	(2,989)	(34,737)
Interest rate swap agreements
Derivative [Line Items]
Aggregate notional amount	$ 2,281,000		2,281,000		$ 1,771,000
Number of new swap agreements | item	16
Number of matured interest rate swap agreements | item	2
Number of terminated interest rate swap agreements | item	5
Notional amount of swap agreements matured during period	$ 100,000
Notional amount of swap agreements terminated during period	450,000
Derivative notional amount of interest rate swap agreements	$ 995,000		$ 995,000
Interest rate swap agreements | Minimum
Derivative [Line Items]
Fixed interest rate	0.7615%		0.7615%
Interest rate swap agreements | Maximum
Derivative [Line Items]
Fixed interest rate	2.182%		2.182%
Eurodollar Future
Derivative [Line Items]
Interest rate swap agreements, weighted average maturity (in years)			3 months
Notional value of euro dollar future contract	$ 1,000		$ 1,000
Number of futures contracts | derivative					1,250
Derivative notional amount futures contracts					$ 1,250,000
Gain on derivatives		(2,060)	262	(3,396)
TBA Agency MBS
Derivative [Line Items]
Aggregate notional amount	750,000		750,000
Gain on derivatives	$ (2,475)	$ 3,412	$ 706	$ 26,826
Term of TBA Agency MBS			15 years
TBA Agency MBS | Minimum
Derivative [Line Items]
Fixed interest rate	2.50%		2.50%
TBA Agency MBS | Maximum
Derivative [Line Items]
Fixed interest rate	3.00%		3.00%