Derivative Instruments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) Derivative	Dec. 31, 2015 USD ($) Derivative	Dec. 31, 2014 USD ($)
Derivative [Line Items]
Interest rate swap agreements, weighted average maturity (in years)	34 months
Number of matured or terminated interest rate swap agreements | Derivative	28
Notional amount of swap agreements matured or terminated during period	$ 2,220,000,000
Number of new swap agreements | Derivative	3
Notional amount of swap agreements added during period	$ 125,000,000,000
Swap agreements, maximum maturity term	5 years
AOCI relating to interest rate swaps	$ 17,800,000
Interest rate cash flow hedge gain (loss) to be reclassified during next 12 months	2,400,000
Interest rate swap agreements
Derivative [Line Items]
Aggregate notional amount	$ 1,771,000,000	$ 3,866,000,000
Interest rate swap agreements | Minimum
Derivative [Line Items]
Fixed interest rate	0.598%
Interest rate swap agreements | Maximum
Derivative [Line Items]
Fixed interest rate	3.06%
Eurodollar Future
Derivative [Line Items]
Interest rate swap agreements, weighted average maturity (in years)	3 months
Notional value of euro dollar future contract	$ 1,000,000
Number of futures contracts | Derivative	1,250	4,550
Derivative notional amount futures contracts	$ 1,250,000,000	$ 4,550,000,000
Gain (loss) on derivatives	(2,723,000)	(2,103,000)	$ (1,694,000)
TBA Agency MBS
Derivative [Line Items]
Aggregate notional amount	605,000,000
Gain (loss) on derivatives	$ 11,464,000	$ 9,323,000	$ 10,228,000
Term of TBA Agency MBS	15 years
TBA Agency MBS | Minimum
Derivative [Line Items]
Fixed interest rate	2.50%
TBA Agency MBS | Maximum
Derivative [Line Items]
Fixed interest rate	3.00%