Offsetting of Derivative Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Interest Rate Swaps
Offsetting Assets [Line Items]
Gross Amounts, Assets	$ 340,890	$ 559,044
Amounts Eligible for Offset, Assets	(311,264)	(408,553)
Net Amounts, Assets	29,626	150,491
Gross Amounts, Liabilities	1,272,616	1,141,828
Amounts Eligible for Offset, Liabilities	(311,264)	(408,553)
Net Amounts, Liabilities	961,352	733,275
Interest Rate Swaption
Offsetting Assets [Line Items]
Gross Amounts, Assets	36,965	110,361
Amounts Eligible for Offset, Assets		(24,662)
Net Amounts, Assets	36,965	85,699
Gross Amounts, Liabilities		24,662
Amounts Eligible for Offset, Liabilities		(24,662)
TBA Derivatives
Offsetting Assets [Line Items]
Gross Amounts, Assets	3,140	20,693
Amounts Eligible for Offset, Assets	(1,304)	(9,775)
Net Amounts, Assets	1,836	10,918
Gross Amounts, Liabilities	6,045	13,779
Amounts Eligible for Offset, Liabilities	(1,304)	(9,775)
Net Amounts, Liabilities	4,741	4,004
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
Us Treasury futures
Offsetting Assets [Line Items]
Gross Amounts, Assets		3,487
Amounts Eligible for Offset, Assets		(439)
Net Amounts, Assets		3,048
Gross Amounts, Liabilities		439
Amounts Eligible for Offset, Liabilities		$ (439)