Offsetting of Derivative Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Interest Rate Swaps
Offsetting Assets [Line Items]
Gross Amounts, Assets	$ 559,044	$ 26,020
Amounts Eligible for Offset, Assets	(408,553)	(26,020)
Net Amounts, Assets	150,491
Gross Amounts, Liabilities	1,141,828	2,610,927
Amounts Eligible for Offset, Liabilities	(408,553)	(26,020)
Net Amounts, Liabilities	733,275	2,584,907
TBA Derivatives
Offsetting Assets [Line Items]
Gross Amounts, Assets	20,693	1,875
Amounts Eligible for Offset, Assets	(9,775)
Net Amounts, Assets	10,918	1,875
Gross Amounts, Liabilities	13,779
Amounts Eligible for Offset, Liabilities	(9,775)
Net Amounts, Liabilities	4,004
Us Treasury futures
Offsetting Assets [Line Items]
Gross Amounts, Assets	3,487	7,955
Amounts Eligible for Offset, Assets	(439)
Net Amounts, Assets	3,048	7,955
Gross Amounts, Liabilities	439
Amounts Eligible for Offset, Liabilities	(439)
Interest Rate Swaption
Offsetting Assets [Line Items]
Gross Amounts, Assets	110,361
Amounts Eligible for Offset, Assets	(24,662)
Net Amounts, Assets	85,699
Gross Amounts, Liabilities	24,662
Amounts Eligible for Offset, Liabilities	(24,662)
Collateralized Mortgage Backed Securities
Offsetting Assets [Line Items]
Gross Amounts, Assets	12,184
Amounts Eligible for Offset, Assets	(3,292)
Net Amounts, Assets	8,892
Gross Amounts, Liabilities	16,638
Amounts Eligible for Offset, Liabilities	(3,292)
Net Amounts, Liabilities	$ 13,346