Offsetting of Derivative Assets and Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Offsetting Assets [Line Items]
Gross Amounts, Liabilities	$ 69,171	$ 49,963	[1]
Future
Offsetting Assets [Line Items]
Gross Amounts, Assets	717	12,443
Amounts Eligible for Offset -Financial Instruments, Assets	(110)	(10,868)
Amounts Eligible for Offset-Cash Collateral, Assets	0	0
Net Amounts, Assets	607	1,575
Gross Amounts, Liabilities	69,171	32,778
Amounts Eligible for Offset-Financial Instruments, Liabilities	(110)	(10,868)
Amounts Eligible for Offset -Cash Collateral, Liabilities	(69,061)	(21,910)
Interest Rate Swaps
Offsetting Assets [Line Items]
Gross Amounts, Assets	93,312	19,642
Amounts Eligible for Offset -Financial Instruments, Assets	(93,312)	(18,040)
Amounts Eligible for Offset-Cash Collateral, Assets	0	0
Net Amounts, Assets		1,602
Gross Amounts, Liabilities	2,782,961	1,677,571
Amounts Eligible for Offset-Financial Instruments, Liabilities	(93,312)	(18,040)
Amounts Eligible for Offset -Cash Collateral, Liabilities	(1,617,530)	(913,576)
Net Amounts, Liabilities	1,072,119	745,955
TBA Derivatives
Offsetting Assets [Line Items]
Gross Amounts, Assets	76,732
Amounts Eligible for Offset-Cash Collateral, Assets	0
Net Amounts, Assets	$ 76,732
Gross Amounts, Liabilities		17,185
Amounts Eligible for Offset-Financial Instruments, Liabilities		(7,367)
Net Amounts, Liabilities		9,818
Interest Rate Swaption
Offsetting Assets [Line Items]
Gross Amounts, Assets		9,622
Amounts Eligible for Offset -Financial Instruments, Assets		(7,367)
Amounts Eligible for Offset-Cash Collateral, Assets		0
Net Amounts, Assets		$ 2,255

[1] Derived from the audited consolidated financial statements at December 31, 2015.