Derivative Instruments - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	1 Months Ended	3 Months Ended	9 Months Ended			3 Months Ended	9 Months Ended
	Sep. 30, 2014 Derivative	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Mar. 17, 2014 Not Designated as Hedging Instrument	Sep. 30, 2014 TBA Securities - Open Contract	Sep. 30, 2014 TBA Securities - Open Contract	Sep. 30, 2014 TBA Securities - Open Contract TBA Securities	Sep. 30, 2014 TBA Securities - Open Contract TBA Securities	Sep. 30, 2014 Interest rate swap agreements Derivative	Sep. 30, 2014 Interest rate swap agreements Derivative	Sep. 30, 2014 Interest rate swap agreements	Mar. 17, 2014 Interest rate swap agreements	Dec. 31, 2013 Interest rate swap agreements	Sep. 30, 2014 Eurodollar Futures Contracts Derivative	Sep. 30, 2014 Eurodollar Futures Contracts Derivative	Sep. 30, 2014 New Swap Agreements Interest rate swap agreements Derivative
Derivative [Line Items]
Interest rate swap agreements, aggregate notional amount						$ 1,685,000,000					$ 3,805,000,000	$ 3,805,000,000	$ 3,805,000,000	$ 1,685,000,000	$ 5,375,000,000	$ 1,000,000	$ 1,000,000	$ 350,000,000
Interest rate swap agreements, weighted average maturity (in years)			4 years														3 months
Number of interest rate swap agreements																		4
Number of interest swaps matured	0
Number of interest rate swap agreements, terminated											32	32
Notional balance of terminated interest rate swap agreements												1,810,000,000
Fixed interest rate during term of swap agreements, lower range	0.578%		0.578%				2.50%	2.50%
Fixed interest rate during term of swap agreements, higher range	3.06%		3.06%				3.50%	3.50%
Increase/Decrease in unrealized losses of swap agreements included in other comprehensive income			20,600,000
Unrealized losses of swap agreements included in other comprehensive income			53,900,000		33,300,000
Unrealized gains (losses) on derivatives	987,000	(40,863,000)	(73,959,000)	(10,715,000)
Reclassification adjustment for interest expense included in net income	13,382,000	14,633,000	53,372,000	38,426,000
Gain on derivatives-TBA securities	(413,000)		1,164,000						779,000	798,000
Derivative income-TBA securities							366,000	366,000
Maximum length of swap agreements								15 years
Interest rate swap agreement, notional amount terminated											1,810,000,000
Net gain (loss) on interest rate swaps	10,947,000		9,568,000									10,900,000	9,600,000			111,000	111,000
Number of futures contracts																5,500	5,500
Fair value of futures contracts																$ 1,400,000	$ 1,400,000