Financial Instruments - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 77,942	$ 98,294	$ 105,357	$ 135,366
ASSETS
Cash equivalents	33,364	60,268
Securities segregated for regulatory and other purposes		11,495
Deposits with clearing organizations	16,595	10,492
Securities owned:
Securities Owned	806,772	856,088
Investments	7,718
Loans held for sale	20,095	75,989
Securities purchased under agreements to resell	250,000	184,000
Derivative contracts:
Derivative contracts, total	7,285	4,530
Total	1,192,229	1,267,309
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	128,262	76,314
Investments		648
Derivative contracts:
Derivative contracts	4,542	6,574
Total	132,804	83,536
Federal Funds Futures [Member]
Derivative contracts:
Derivative contracts	135	18
Eurodollar Future [Member]
Derivative contracts:
Derivative contracts	60	44
general collateral futures [Member]
Derivative contracts:
Derivative contracts	2
To Be Announced Security [Member]
Derivative contracts:
Derivative contracts, total	1,774	2,155
Derivative contracts:
TBAs	33	73
Interest Rate Lock Commitments [Member]
Derivative contracts:
Derivative contracts, total	5,511	2,375
Derivative contracts:
Derivative contracts	3,188	3,653
U.S. Treasury Futures [Member]
Derivative contracts:
Derivative contracts	87	186
Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	1,037	2,600
Corporate Equities [Member]
Securities owned:
Securities Owned	49,012	61,634
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	49,457	45,336
Money Markets [Member]
Securities owned:
Securities Owned	2,668	1,263
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	15	241
Auction Rate Securities [Member]
Securities owned:
Securities Owned	87,323	85,124
Investments Included in Other Assets [Member]
Securities owned:
Investments	58,118	64,447
U.S. Treasury Securities [Member]
Securities owned:
Securities Owned	503,509	566,346
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	67,472	11,837
U.S. Agency Securities [Member]
Securities owned:
Securities Owned	15,067	29,448
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	24	52
Sovereign Obligations [Member]
Securities owned:
Securities Owned	3,434	320
Corporate Debt and Other Obligations [Member]
Securities owned:
Securities Owned	19,922	14,673
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	3,010	4,847
Mortgage and Other Asset-Backed Securities [Member]
Securities owned:
Securities Owned	4,792	3,395
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value		7
Municipal Obligations [Member]
Securities owned:
Securities Owned	62,250	40,166
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	55	72
Convertible Bonds [Member]
Securities owned:
Securities Owned	58,795	53,719
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	8,229	13,922
Level 1 [Member]
ASSETS
Cash equivalents	33,364	60,268
Securities segregated for regulatory and other purposes		11,495
Deposits with clearing organizations	16,595	10,492
Securities owned:
Securities Owned	555,189	629,243
Derivative contracts:
Total	605,246	722,273
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	116,944	57,414
Investments		648
Derivative contracts:
Derivative contracts	284	248
Total	117,228	58,310
Level 1 [Member] | Federal Funds Futures [Member]
Derivative contracts:
Derivative contracts	135	18
Level 1 [Member] | Eurodollar Future [Member]
Derivative contracts:
Derivative contracts	60	44
Level 1 [Member] | general collateral futures [Member]
Derivative contracts:
Derivative contracts	2
Level 1 [Member] | U.S. Treasury Futures [Member]
Derivative contracts:
Derivative contracts	87
Level 1 [Member] | U.S. Treasury Futures [Member]
Derivative contracts:
Derivative contracts		186
Level 1 [Member] | Corporate Equities [Member]
Securities owned:
Securities Owned	49,012	61,634
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	49,457	45,336
Level 1 [Member] | Money Markets [Member]
Securities owned:
Securities Owned	2,668	1,263
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	15	241
Level 1 [Member] | Investments Included in Other Assets [Member]
Securities owned:
Investments	98	10,775
Level 1 [Member] | U.S. Treasury Securities [Member]
Securities owned:
Securities Owned	503,509	566,346
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	67,472	11,837
Level 2 [Member]
Securities owned:
Securities Owned	164,131	141,485
Loans held for sale	20,095	75,989
Securities purchased under agreements to resell	250,000	184,000
Derivative contracts:
Derivative contracts, total	1,774	2,155
Total	485,597	451,355
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	11,318	18,900
Derivative contracts:
Derivative contracts	33	73
Total	11,351	18,973
Level 2 [Member] | Eurodollar Future [Member]
Derivative contracts:
Derivative contracts	0
Level 2 [Member] | To Be Announced Security [Member]
Derivative contracts:
Derivative contracts, total	1,774	2,155
Derivative contracts:
TBAs	33	73
Level 2 [Member] | Federal Funds Futures [Member]
Derivative contracts:
Derivative contracts	0
Level 2 [Member] | Investments Included in Other Assets [Member]
Securities owned:
Investments	49,597	47,726
Level 2 [Member] | U.S. Agency Securities [Member]
Securities owned:
Securities Owned	15,067	29,448
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	24	52
Level 2 [Member] | Sovereign Obligations [Member]
Securities owned:
Securities Owned	3,434	320
Level 2 [Member] | Corporate Debt and Other Obligations [Member]
Securities owned:
Securities Owned	19,922	14,673
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	3,010	4,847
Level 2 [Member] | Mortgage and Other Asset-Backed Securities [Member]
Securities owned:
Securities Owned	4,792	3,395
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value		7
Level 2 [Member] | Municipal Obligations [Member]
Securities owned:
Securities Owned	62,121	39,930
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	55	72
Level 2 [Member] | Convertible Bonds [Member]
Securities owned:
Securities Owned	58,795	53,719
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	8,229	13,922
Level 3 [Member]
Securities owned:
Securities Owned	87,452	85,360
Derivative contracts:
Derivative contracts, total	5,511	2,375
Total	101,386	93,681
Derivative contracts:
Derivative contracts	4,225	6,253
Total	4,225	6,253
Level 3 [Member] | Interest Rate Lock Commitments [Member]
Derivative contracts:
Derivative contracts, total	5,511	2,375
Derivative contracts:
Derivative contracts	3,188	3,653
Level 3 [Member] | Auction Rate Securities [Member]
Securities owned:
Securities Owned	87,323	85,124
Level 3 [Member] | Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	1,037	2,600
Level 3 [Member] | Investments Included in Other Assets [Member]
Securities owned:
Investments	8,423	5,946
Level 3 [Member] | Municipal Obligations [Member]
Securities owned:
Securities Owned	$ 129	$ 236