Offsetting of Derivative Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Offsetting Assets [Line Items]
Gross Amounts, Liabilities	$ 61,778	$ 8,027	[1]
Future
Offsetting Assets [Line Items]
Gross Amounts, Assets		117
Amounts Eligible for Offset -Financial Instruments, Assets		(117)
Amounts Eligible for Offset-Cash Collateral, Assets		0
Gross Amounts, Liabilities	61,778	3,769
Amounts Eligible for Offset-Financial Instruments, Liabilities		(117)
Amounts Eligible for Offset -Cash Collateral, Liabilities	(61,778)
Net Amounts, Liabilities		3,652
Interest Rate Swaps
Offsetting Assets [Line Items]
Gross Amounts, Assets	25,908	75,225
Amounts Eligible for Offset -Financial Instruments, Assets	(24,782)	(66,180)
Amounts Eligible for Offset-Cash Collateral, Assets	0	0
Net Amounts, Assets	1,126	9,045
Gross Amounts, Liabilities	2,025,170	1,608,286
Amounts Eligible for Offset-Financial Instruments, Liabilities	24,782	(66,180)
Amounts Eligible for Offset -Cash Collateral, Liabilities	(1,111,199)	(869,302)
Net Amounts, Liabilities	889,189	672,804
TBA Derivatives
Offsetting Assets [Line Items]
Gross Amounts, Assets	112,929
Amounts Eligible for Offset-Cash Collateral, Assets	0
Net Amounts, Assets	112,929
Gross Amounts, Liabilities		4,258
Net Amounts, Liabilities		4,258
Interest Rate Swaption
Offsetting Assets [Line Items]
Gross Amounts, Assets	574	5,382
Amounts Eligible for Offset-Cash Collateral, Assets	0	0
Net Amounts, Assets	$ 574	$ 5,382

[1] Derived from the audited consolidated financial statements at December 31, 2014.