Derivative Instruments - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) derivative
Derivative [Line Items]
Aggregate notional amount	$ 1,836,000		$ 1,836,000		$ 1,771,000
Interest rate swap agreements, weighted average maturity (in years)			37 months
AOCI relating to interest rate swaps			$ 16,600
Interest rate cash flow hedge gain (loss) to be reclassified during next 12 months	3,000		3,000
Gain (loss) on interest rate swaps, net	(5,960)	$ (16,089)	(5,487)	$ (66,308)
Gain on derivatives	(4,422)	(9,385)	(2,044)	(44,228)
Interest rate swap agreements
Derivative [Line Items]
Aggregate notional amount	$ 1,836,000		$ 1,836,000		$ 1,771,000
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	3.06%		3.06%
Eurodollar Future
Derivative [Line Items]
Interest rate swap agreements, weighted average maturity (in years)			3 months
Notional amount of swap agreements matured during period			$ 175,000
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	(14)	(1,357)	262	(1,335)
TBA Agency MBS
Derivative [Line Items]
Aggregate notional amount	755,000		755,000
Gain on derivatives	$ 1,552	$ 8,061	$ 3,181	$ 23,415
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%