Fair Value Measurements - Changes in Level 3 Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Other Assets and Other Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 17,347	$ 3,941	$ 47,859
Total gains (losses) realized/unrealized:
Included in earnings	87,061	83,417	97,845
Transfers in and/or out of Level 3	(94,529)	(70,011)	(141,763)
Ending Balance	9,879	17,347	3,941
Changes in unrealized gains (losses) included in earnings related to assets still held at end of period	8,850	18,196	3,431
Privately Issued [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	103	1,850	1,023,886
Total gains (losses) realized/unrealized:
Included in earnings			(56,102)
Included in other comprehensive income		271	116,359
Sales			(978,608)
Settlements	(29)	(2,018)	(103,685)
Ending Balance	74	103	1,850
Collateralized Debt Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	50,316	63,083	61,869
Total gains (losses) realized/unrealized:
Included in other comprehensive income	3,254	8,209	4,508
Settlements	(6,177)	(20,976)	(3,294)
Ending Balance	$ 47,393	$ 50,316	$ 63,083