Financial Instruments - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 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	$ 65,174	$ 98,294	$ 106,098	$ 135,366
ASSETS
Cash equivalents	28,364	60,268
Securities segregated for regulatory and other purposes		11,495
Deposits with clearing organizations	16,091	10,492
Securities owned:
Securities Owned	944,471	856,088
Investments	8,049
Loans held for sale	15,806	75,989
Securities purchased under agreements to resell	250,000	184,000
Derivative contracts:
Derivative contracts, total	12,423	4,530
Total	1,326,368	1,267,309
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	126,092	76,314
Investments	136	648
Derivative contracts:
Derivative contracts	5,847	6,574
Total	132,075	83,536
Euro Dollars Futures [Member]
Derivative contracts:
Derivative contracts	88	44
TBAs [Member]
Derivative contracts:
Derivative contracts, total	1,895	2,155
Derivative contracts:
TBAs	181
Derivative contracts		73
Interest Rate Lock Commitments [Member]
Derivative contracts:
Derivative contracts, total	10,528	2,375
Derivative contracts:
TBAs		3,653
Derivative contracts	3,833
U.S. Treasury Futures [Member]
Derivative contracts:
Derivative contracts	188	186
Federal Funds Futures [Member]
Derivative contracts:
Derivative contracts	43	18
Corporate Equities [Member]
Securities owned:
Securities Owned	54,629	61,634
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	46,116	45,336
Money Markets [Member]
Securities owned:
Securities Owned	2,647	1,263
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	26	241
Auction Rate Securities [Member]
Securities owned:
Securities Owned	92,548	85,124
Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	1,514	2,600
Investments Included in Other Assets [Member]
Securities owned:
Investments	59,213	64,447
U.S. Treasury Securities [Member]
Securities owned:
Securities Owned	614,757	566,346
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	71,693	11,837
U.S. Agency Securities [Member]
Securities owned:
Securities Owned	26,167	29,448
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	35	52
Sovereign Obligations [Member]
Securities owned:
Securities Owned	1,201	320
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	599
Corporate Debt and Other Obligations [Member]
Securities owned:
Securities Owned	23,791	14,673
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	477	4,847
Mortgage and Other Asset-Backed Securities [Member]
Securities owned:
Securities Owned	4,914	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	63,947	40,166
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	73	72
Convertible Bonds [Member]
Securities owned:
Securities Owned	59,870	53,719
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	7,073	13,922
Level 1 [Member]
ASSETS
Cash equivalents	28,364	60,268
Securities segregated for regulatory and other purposes		11,495
Deposits with clearing organizations	16,091	10,492
Securities owned:
Securities Owned	672,033	629,243
Derivative contracts:
Total	716,896	722,273
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	117,835	57,414
Investments	136	648
Derivative contracts:
Derivative contracts	188	186
Total	118,159	58,248
Level 1 [Member] | U.S. Treasury Futures [Member]
Derivative contracts:
Derivative contracts		186
Level 1 [Member] | Corporate Equities [Member]
Securities owned:
Securities Owned	54,629	61,634
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	46,116	45,336
Level 1 [Member] | Money Markets [Member]
Securities owned:
Securities Owned	2,647	1,263
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	26	241
Level 1 [Member] | Investments Included in Other Assets [Member]
Securities owned:
Investments	408	10,775
Level 1 [Member] | U.S. Treasury Securities [Member]
Securities owned:
Securities Owned	614,757	566,346
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	71,693	11,837
Level 2 [Member]
Securities owned:
Securities Owned	179,838	141,485
Loans held for sale	15,806	75,989
Securities purchased under agreements to resell	250,000	184,000
Derivative contracts:
Derivative contracts, total	1,895	2,155
Total	497,565	451,355
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	8,257	18,900
Derivative contracts:
Derivative contracts	312	135
Total	8,569	19,035
Level 2 [Member] | Euro Dollars Futures [Member]
Derivative contracts:
Derivative contracts	88	44
Level 2 [Member] | TBAs [Member]
Derivative contracts:
Derivative contracts, total	1,895	2,155
Derivative contracts:
TBAs	181	73
Level 2 [Member] | Federal Funds Futures [Member]
Derivative contracts:
Derivative contracts	43	18
Level 2 [Member] | Investments Included in Other Assets [Member]
Securities owned:
Investments	50,026	47,726
Level 2 [Member] | U.S. Agency Securities [Member]
Securities owned:
Securities Owned	26,167	29,448
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	35	52
Level 2 [Member] | Sovereign Obligations [Member]
Securities owned:
Securities Owned	1,201	320
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	599
Level 2 [Member] | Corporate Debt and Other Obligations [Member]
Securities owned:
Securities Owned	23,791	14,673
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	477	4,847
Level 2 [Member] | Mortgage and Other Asset-Backed Securities [Member]
Securities owned:
Securities Owned	4,914	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	63,895	39,930
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	73	72
Level 2 [Member] | Convertible Bonds [Member]
Securities owned:
Securities Owned	59,870	53,719
Securities sold, but not yet purchased:
Securities sold, but not yet purchased, at fair value	7,073	13,922
Level 3 [Member]
Securities owned:
Securities Owned	92,600	85,360
Derivative contracts:
Derivative contracts, total	10,528	2,375
Total	111,907	93,681
Derivative contracts:
Derivative contracts	5,347	6,253
Total	5,347	6,253
Level 3 [Member] | Interest Rate Lock Commitments [Member]
Derivative contracts:
Derivative contracts, total	10,528	2,375
Derivative contracts:
Derivative contracts	3,833	3,653
Level 3 [Member] | Auction Rate Securities [Member]
Securities owned:
Securities Owned	92,548	85,124
Level 3 [Member] | Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	1,514	2,600
Level 3 [Member] | Investments Included in Other Assets [Member]
Securities owned:
Investments	8,779	5,946
Level 3 [Member] | Municipal Obligations [Member]
Securities owned:
Securities Owned	$ 52	$ 236