Financial instruments - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 50,909	$ 58,945
Securities segregated for regulatory and other purposes	17,092	11,499
Deposits with clearing organizations	3,499	9,095
Securities owned
Securities owned, at fair value	848,329	759,742
Investments	11,919
TBAs	3,111	3,188
Total	978,477	902,988
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	63,046	173,450
Investments	270	258
Derivative contracts	2,525	3,057
TBAs	281	175
Total	66,122	176,940
Corporate equities [Member]
Securities owned
Securities owned, at fair value	42,091	43,708
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	34,401	29,884
Other [Member]
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	114	425
U.S. Treasury obligations [Member]
Securities owned
Securities owned, at fair value	571,923	497,546
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	12,290	131,899
U.S. Agency obligations [Member]
Securities owned
Securities owned, at fair value	39,584	27,690
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	30	31
Sovereign obligations [Member]
Securities owned
Securities owned, at fair value	803	19
Corporate debt and other obligations [Member]
Securities owned
Securities owned, at fair value	20,577	14,428
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	6,811	1,858
Mortgage and other asset-backed securities [Member]
Securities owned
Securities owned, at fair value	3,385	2,920
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	25	18
Municipal obligations [Member]
Securities owned
Securities owned, at fair value	61,551	59,010
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	556	467
Convertible bonds [Member]
Securities owned
Securities owned, at fair value	43,871	49,130
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	8,819	8,868
Other [Member]
Securities owned
Securities owned, at fair value	64,544	65,291
Other [Member]
Securities owned
Investments	55,537	60,519
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	50,909	58,945
Securities segregated for regulatory and other purposes	17,092	11,499
Deposits with clearing organizations	3,499	9,095
Securities owned
Securities owned, at fair value	617,744	534,291
TBAs
Total	690,180	624,307
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	30,156	153,170
Investments	270	258
Derivative contracts	360	286
TBAs
Total	30,786	153,714
Level 1 [Member] | Corporate equities [Member]
Securities owned
Securities owned, at fair value	30,333	31,958
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	17,852	20,946
Level 1 [Member] | Other [Member]
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	14	325
Level 1 [Member] | U.S. Treasury obligations [Member]
Securities owned
Securities owned, at fair value	571,923	497,546
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	12,290	131,899
Level 1 [Member] | U.S. Agency obligations [Member]
Securities owned
Securities owned, at fair value	10,859
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value
Level 1 [Member] | Sovereign obligations [Member]
Securities owned
Securities owned, at fair value
Level 1 [Member] | Corporate debt and other obligations [Member]
Securities owned
Securities owned, at fair value	2,574	2,459
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value
Level 1 [Member] | Mortgage and other asset-backed securities [Member]
Securities owned
Securities owned, at fair value
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value
Level 1 [Member] | Municipal obligations [Member]
Securities owned
Securities owned, at fair value
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value
Level 1 [Member] | Convertible bonds [Member]
Securities owned
Securities owned, at fair value
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value
Level 1 [Member] | Other [Member]
Securities owned
Securities owned, at fair value	2,055	2,328
Level 1 [Member] | Other [Member]
Securities owned
Investments	936	10,477
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents
Securities segregated for regulatory and other purposes
Deposits with clearing organizations
Securities owned
Securities owned, at fair value	157,741	153,054
TBAs	3,111	3,188
Total	202,674	193,330
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	32,790	20,180
Investments
Derivative contracts	71	124
TBAs	281	175
Total	33,142	20,479
Level 2 [Member] | Corporate equities [Member]
Securities owned
Securities owned, at fair value	11,758	11,750
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	16,549	8,938
Level 2 [Member] | Other [Member]
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value
Level 2 [Member] | U.S. Treasury obligations [Member]
Securities owned
Securities owned, at fair value
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value
Level 2 [Member] | U.S. Agency obligations [Member]
Securities owned
Securities owned, at fair value	28,725	27,690
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	30	31
Level 2 [Member] | Sovereign obligations [Member]
Securities owned
Securities owned, at fair value	803	19
Level 2 [Member] | Corporate debt and other obligations [Member]
Securities owned
Securities owned, at fair value	18,003	11,969
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	6,811	1,858
Level 2 [Member] | Mortgage and other asset-backed securities [Member]
Securities owned
Securities owned, at fair value	3,333	2,880
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	25	18
Level 2 [Member] | Municipal obligations [Member]
Securities owned
Securities owned, at fair value	51,248	49,616
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	556	467
Level 2 [Member] | Convertible bonds [Member]
Securities owned
Securities owned, at fair value	43,871	49,130
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	8,819	8,868
Level 2 [Member] | Other [Member]
Securities owned
Securities owned, at fair value
Level 2 [Member] | Other [Member]
Securities owned
Investments	41,822	37,088
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents
Securities segregated for regulatory and other purposes
Deposits with clearing organizations
Securities owned
Securities owned, at fair value	72,844	72,397
TBAs
Total	85,623	85,351
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	100	100
Investments
Derivative contracts	2,094	2,647
TBAs
Total	2,194	2,747
Level 3 [Member] | Corporate equities [Member]
Securities owned
Securities owned, at fair value
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value
Level 3 [Member] | Other [Member]
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value	100	100
Level 3 [Member] | U.S. Treasury obligations [Member]
Securities owned
Securities owned, at fair value
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value
Level 3 [Member] | U.S. Agency obligations [Member]
Securities owned
Securities owned, at fair value
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value
Level 3 [Member] | Sovereign obligations [Member]
Securities owned
Securities owned, at fair value
Level 3 [Member] | Corporate debt and other obligations [Member]
Securities owned
Securities owned, at fair value
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value
Level 3 [Member] | Mortgage and other asset-backed securities [Member]
Securities owned
Securities owned, at fair value	52	40
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value
Level 3 [Member] | Municipal obligations [Member]
Securities owned
Securities owned, at fair value	10,303	9,394
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value
Level 3 [Member] | Convertible bonds [Member]
Securities owned
Securities owned, at fair value
Securities sold, but not yet purchased
Securities sold, but not yet purchased at fair value
Level 3 [Member] | Other [Member]
Securities owned
Securities owned, at fair value	62,489	62,963
Level 3 [Member] | Other [Member]
Securities owned
Investments	$ 12,779	$ 12,954