Credit Risk and Interest Rate Risk - Offsetting Assets and Liabilities (Detail) - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014
Offsetting Assets and Liabilities [Line Items]
Repurchase Agreements, Gross Amounts of Recognized Assets (Liabilities)		$ (7,439,339)	$ (8,455,381)
Repurchase Agreements, Gross Amounts Offset in the Consolidated Statements of Financial Position		0	0
Repurchase Agreements, Net Amounts Offset in the Consolidated Statements of Financial Position		(7,439,339)	(8,455,381)
Repurchase Agreements, Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Financial Instruments		8,773,285	9,309,738
Repurchase Agreements, Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Cash Collateral (Received) Pledged	[1]	37,296
Repurchase Agreements, Net Amount		1,371,242	854,357
Total, Gross Amounts of Recognized Assets (Liabilities)		(7,475,795)	(8,573,432)
Total, Gross Amounts Offset in the Consolidated Statements of Financial Position		42,282	106,763
Total, Net Amounts Offset in the Consolidated Statements of Financial Position		(7,433,513)	(8,466,669)
Total, Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Financial Instruments		8,781,874	9,329,078
Total, Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Cash Collateral (Received) Pledged	[1]	95,140	77,391
Total, Net Amount		1,443,501	939,800
Derivative Assets, Net Amounts Offset in the Consolidated Statements of Financial Position		15,460	3,631
Derivative Liabilities, Net Amounts Offset in the Consolidated Statements of Financial Position		(9,634)	(14,177)
Interest Rate Swaps [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative Assets, Gross Amounts of Recognized Assets		6,166
Derivative Assets, Gross Amounts Offset in the Consolidated Statements of Financial Position		(4,934)
Derivative Assets, Net Amounts Offset in the Consolidated Statements of Financial Position		1,232
Derivative Assets, Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Cash Collateral (Received) Pledged		0
Derivative Assets, Net Amount		1,232
Derivative Liabilities, Gross Amounts of Recognized Liabilities		(53,777)	(113,597)
Derivative Liabilities, Gross Amounts Offset in the Consolidated Statements of Financial Position		47,408	99,536
Derivative Liabilities, Net Amounts Offset in the Consolidated Statements of Financial Position		(6,369)	(14,061)
Derivative Liabilities, Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Financial Instruments		8,589	19,340
Derivative Liabilities, Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Cash Collateral (Received) Pledged	[1]	51,424	64,796
Derivative Liabilities, Net Amount		53,644	70,075
Swaptions [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative Assets, Gross Amounts of Recognized Assets		12,821	2,889
Derivative Assets, Net Amounts Offset in the Consolidated Statements of Financial Position		12,821	2,889
Derivative Assets, Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Cash Collateral (Received) Pledged		0	0
Derivative Assets, Net Amount		12,821	2,889
Derivative Liabilities, Gross Amounts of Recognized Liabilities		(3,265)	(45)
Derivative Liabilities, Net Amounts Offset in the Consolidated Statements of Financial Position		(3,265)	(45)
Derivative Liabilities, Net Amount		(3,265)	(45)
Treasury Futures [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative Assets, Gross Amounts of Recognized Assets		1,599
Derivative Assets, Gross Amounts Offset in the Consolidated Statements of Financial Position		(192)
Derivative Assets, Net Amounts Offset in the Consolidated Statements of Financial Position		1,407
Derivative Assets, Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Cash Collateral (Received) Pledged		0
Derivative Assets, Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Financial Instruments	[1]	6,420
Derivative Assets, Net Amount		$ 7,827
Derivative Liabilities, Gross Amounts of Recognized Liabilities			(7,227)
Derivative Liabilities, Gross Amounts Offset in the Consolidated Statements of Financial Position			7,227
Derivative Liabilities, Gross Amounts Not Offset with Financial Assets (Liabilities) in the Consolidated Statements of Financial Position, Cash Collateral (Received) Pledged	[1]		12,595
Derivative Liabilities, Net Amount			12,595
Mortgage Options [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative Liabilities, Gross Amounts of Recognized Liabilities			(71)
Derivative Liabilities, Net Amounts Offset in the Consolidated Statements of Financial Position			(71)
Derivative Liabilities, Net Amount			$ (71)

[1] Included in other assets