Fair Value Measurements - Changes in Level 3 Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Other Assets and Other Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 11,206	$ 22,023	$ 17,347	$ 3,941
Total gains (losses) realized/unrealized:
Included in earnings	21,709	9,657	67,611	63,557
Transfers in and/or out of Level 3	(15,268)	(15,188)	(67,311)	(51,006)
Ending Balance	17,647	16,492	17,647	16,492
Changes in unrealized gains included in earnings related to assets still held at end of period	15,488	12,421	15,965	17,773
Privately Issued [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	88	119	103	1,850
Total gains (losses) realized/unrealized:
Included in other comprehensive income				272
Settlements	(6)	(7)	(21)	(2,010)
Ending Balance	82	112	82	112
Collateralized Debt Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	50,483	56,200	50,316	63,083
Total gains (losses) realized/unrealized:
Included in other comprehensive income	(472)	2,201	5,153	11,333
Settlements	(135)	(3,593)	(5,593)	(19,608)
Ending Balance	$ 49,876	$ 54,808	$ 49,876	$ 54,808