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
	Mar. 31, 2016	Mar. 31, 2015
Other Assets and Other Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 9,879	$ 17,347
Total gains (losses) realized/unrealized:
Included in earnings	23,898	29,770
Transfers in and/or out of Level 3	(16,892)	(20,887)
Ending Balance	16,885	26,230
Changes in unrealized gains (losses) included in earnings related to assets still held at end of period	14,539	22,636
Privately Issued [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	74	103
Total gains (losses) realized/unrealized:
Settlements	(9)	(8)
Ending Balance	65	95
Collateralized Debt Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	47,393	50,316
Total gains (losses) realized/unrealized:
Included in other comprehensive income	(2,148)	(2,004)
Settlements	(205)	(1,034)
Ending Balance	$ 45,040	$ 47,278