Fair Value Measurements - Changes in Level 3 Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Total gains (losses) realized/unrealized:
Included in other comprehensive income
Sales
Settlements
Other assets and other liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	7,408	34,051	47,859	6,923
Total gains (losses) realized/unrealized:
Included in earnings	24,440	84,864	83,252	157,381
Transfers in and/or out of Level 3	(18,773)	(48,570)	(118,036)	(93,959)
Ending Balance	13,075	70,345	13,075	70,345
Changes in unrealized gains (losses) included in earnings related to assets still held at end of period	(1,727)	63,357	925	70,187
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	5,272	1,080,519	1,023,886	1,151,285
Total gains (losses) realized/unrealized:
Included in earnings		(5,672)	(56,102)	(27,976)
Included in other comprehensive income	400	40,603	116,984	89,397
Sales			(978,608)
Settlements	(1,856)	(49,814)	(102,344)	(147,070)
Transfers in and/or out of Level 3
Ending Balance	3,816	1,065,636	3,816	1,065,636
Changes in unrealized gains (losses) included in earnings related to assets still held at end of period		(5,672)		(27,976)
Collateralized Debt Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	59,916	55,098	61,869	52,500
Total gains (losses) realized/unrealized:
Included in earnings
Included in other comprehensive income	213	3,297	(324)	7,008
Settlements	(826)	(1,194)	(2,242)	(2,307)
Transfers in and/or out of Level 3
Ending Balance	59,303	57,201	59,303	57,201
Changes in unrealized gains (losses) included in earnings related to assets still held at end of period

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- Definition

Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)

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- Definition

Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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- Definition

Amount of increase (decrease) of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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- Definition

Amount of purchases, (sales), issuances and (settlements) of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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- Definition

Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

+ References

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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-Publisher FASB

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