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
	Mar. 31, 2014	Mar. 31, 2013
Other Assets and Other Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 3,941	$ 47,859
Total gains (losses) realized/unrealized:
Included in earnings	22,383	43,312
Included in other comprehensive income
Settlements
Transfers in and/or out of Level 3	(13,735)	(55,052)
Ending Balance	12,589	36,119
Changes in unrealized gains (losses) included in earnings related to assets still held at end of period	15,050	31,398
Mortgage-backed Securities, Privately issued [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	1,850	1,023,886
Total gains (losses) realized/unrealized:
Included in earnings		(9,800)
Included in other comprehensive income	67	26,381
Settlements	(1,221)	(47,220)
Transfers in and/or out of Level 3
Ending Balance	696	993,247
Changes in unrealized gains (losses) included in earnings related to assets still held at end of period		(9,800)
Collateralized Debt Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	63,083	61,869
Total gains (losses) realized/unrealized:
Included in earnings
Included in other comprehensive income	4,646	740
Settlements	(5,961)	(891)
Transfers in and/or out of Level 3
Ending Balance	61,768	61,718
Changes in unrealized gains (losses) included in earnings related to assets still held at end of period