Collateralized Transactions - Schedule of Gross Amounts and Offsetting Amounts of Reverse Repurchase Agreements, Repurchase Agreements, Securities Borrowed and Securities Lending Transactions (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Securities Financing Transaction [Line Items]
Net Amounts of Assets Presented on the Balance Sheet	$ 250,000	$ 184,825
Net Amounts of Liabilities Presented on the Balance Sheet	688,310	757,491
Gross Amounts of Recognized Assets	617,323	663,566
Gross Amounts Offset in the Balance Sheet	(99,680)	(204,614)
Net Amounts of Assets Presented on the Balance Sheet	517,643	458,952
Financial Instruments	508,001	449,241
Net Amount	9,642	9,711
Gross Amounts of Recognized Liabilities	952,085	1,173,726
Gross Amounts Offset in the Balance Sheet	(99,680)	(204,614)
Net Amounts of Liabilities Presented on the Balance Sheet	852,405	969,112
Financial Instruments	844,518	957,974
Net Amount	7,887	11,138
Reverse Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Gross Amounts of Recognized Assets	349,680	389,439
Gross Amounts Offset in the Balance Sheet	(99,680)	(204,614)
Net Amounts of Assets Presented on the Balance Sheet	250,000	184,825
Financial Instruments	250,000	183,305
Net Amount	0	1,520
Securities Borrowed [Member]
Securities Financing Transaction [Line Items]
Gross Amounts of Recognized Assets	267,643	274,127
Net Amounts of Assets Presented on the Balance Sheet	267,643	274,127
Financial Instruments	258,001	265,936
Net Amount	9,642	8,191
Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Gross Amounts of Recognized Liabilities	787,990	962,105
Gross Amounts Offset in the Balance Sheet	(99,680)	(204,614)
Net Amounts of Liabilities Presented on the Balance Sheet	688,310	757,491
Financial Instruments	686,659	753,003
Net Amount	1,651	4,488
Securities Loaned [Member]
Securities Financing Transaction [Line Items]
Gross Amounts of Recognized Liabilities	164,095	211,621
Net Amounts of Liabilities Presented on the Balance Sheet	164,095	211,621
Financial Instruments	157,859	204,971
Net Amount	$ 6,236	$ 6,650