Financial Instruments, at Fair Value (Detail) - Schedule of fair value by balance sheet grouping (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Assets:
Corporate equity securities	$ 2,679	$ 5,064
Merchant banking investments	4,645	9,559
Derivatives	3,824	4,767
Fixed income	326	1,093
Other investments	1,332	1,379
Total financial instruments owned	12,806	21,862
Liabilities:
Corporate equity securities and Financial instruments sold, not yet purchased	267	1,643
Derivatives	71	0
Total financial instruments sold, not yet purchased	338	1,643
Fair Value, Inputs, Level 1 [Member]
Assets:
Corporate equity securities	2,665	4,928
Merchant banking investments	0	0
Derivatives	27	20
Fixed income	0	0
Other investments	0	0
Total financial instruments owned	2,692	4,948
Liabilities:
Corporate equity securities and Financial instruments sold, not yet purchased	267	1,643
Derivatives	5
Total financial instruments sold, not yet purchased	272	1,643
Fair Value, Inputs, Level 2 [Member]
Assets:
Corporate equity securities	0	0
Merchant banking investments	0	0
Derivatives	0	0
Fixed income	326	1,093
Other investments	0	0
Total financial instruments owned	326	1,093
Liabilities:
Corporate equity securities and Financial instruments sold, not yet purchased	0	0
Derivatives	0
Total financial instruments sold, not yet purchased	0	0
Fair Value, Inputs, Level 3 [Member]
Assets:
Corporate equity securities	14	136
Merchant banking investments	4,645	9,559
Derivatives	3,797	4,747
Fixed income	0	0
Other investments	1,332	1,379
Total financial instruments owned	9,788	15,821
Liabilities:
Corporate equity securities and Financial instruments sold, not yet purchased	0	0
Derivatives	66
Total financial instruments sold, not yet purchased	$ 66	$ 0