Offsetting of Derivative Assets and Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Offsetting Assets [Line Items]
Gross Amounts, Liabilities	$ 113,626	$ 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	113,626	3,769
Amounts Eligible for Offset-Financial Instruments, Liabilities		(117)
Amounts Eligible for Offset -Cash Collateral, Liabilities	(113,626)
Net Amounts, Liabilities		3,652
Interest Rate Swaps
Offsetting Assets [Line Items]
Gross Amounts, Assets	39,295	75,225
Amounts Eligible for Offset -Financial Instruments, Assets	(39,295)	(66,180)
Amounts Eligible for Offset-Cash Collateral, Assets	0	0
Net Amounts, Assets		9,045
Gross Amounts, Liabilities	2,160,350	1,608,286
Amounts Eligible for Offset-Financial Instruments, Liabilities	(39,295)	(66,180)
Amounts Eligible for Offset -Cash Collateral, Liabilities	(1,254,287)	(869,302)
Net Amounts, Liabilities	866,768	672,804
TBA Derivatives
Offsetting Assets [Line Items]
Gross Amounts, Assets	87,516
Amounts Eligible for Offset-Cash Collateral, Assets	0
Net Amounts, Assets	$ 87,516
Gross Amounts, Liabilities		4,258
Net Amounts, Liabilities		4,258
Interest Rate Swaption
Offsetting Assets [Line Items]
Gross Amounts, Assets		5,382
Amounts Eligible for Offset-Cash Collateral, Assets		0
Net Amounts, Assets		$ 5,382

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