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		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Other Assets and Other Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 16,885	$ 26,230	$ 9,879	$ 17,347
Total gains (losses) realized/unrealized:
Included in earnings	35,430	16,132	59,328	45,902
Sales	0	0
Transfers in and/or out of Level 3	(30,932)	(31,156)	(47,824)	(52,043)
Ending Balance	21,383	11,206	21,383	11,206
Changes in unrealized gains included in earnings related to assets still held at end of period	19,882	6,330	20,661	8,763
Privately Issued [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	65	95	74	103
Total gains (losses) realized/unrealized:
Sales	0	0
Settlements	(8)	(7)	(17)	(15)
Ending Balance	57	88	57	88
Collateralized Debt Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	45,040	47,278	47,393	50,316
Total gains (losses) realized/unrealized:
Included in other comprehensive income	(1,070)	7,629	(3,218)	5,625
Sales	0	0
Settlements	(665)	(4,424)	(870)	(5,458)
Ending Balance	$ 43,305	$ 50,483	$ 43,305	$ 50,483