Summary of Fair Value Information about Derivative Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivative assets	$ 705,769	$ 9,830
Derivative liabilities	1,197,346	2,584,907
Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Derivative assets	559,044
Derivative liabilities	1,141,828	2,584,907
Other Derivative Contracts | Interest Rate Swaption
Derivatives, Fair Value [Line Items]
Derivative assets	110,361
Derivative liabilities	24,662
Other Derivative Contracts | TBA Derivatives
Derivatives, Fair Value [Line Items]
Derivative assets	20,693	1,875
Derivative liabilities	13,779
Other Derivative Contracts | Collateralized Mortgage Backed Securities
Derivatives, Fair Value [Line Items]
Derivative assets	12,184
Other Derivative Contracts | Us Treasury futures
Derivatives, Fair Value [Line Items]
Derivative assets	3,487	7,955
Derivative liabilities	439
Other Derivative Contracts | MBS Options
Derivatives, Fair Value [Line Items]
Derivative liabilities	$ 16,638