Fair Value (Details) (USD $)	3 Months Ended			12 Months Ended
	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2011		Dec. 31, 2010
Securities sold not yet purchased [Abstract]
Balance as of beginning of period			$ 59,414,000	$ 59,414,000		$ 98,559,000
Transfer (out) in, net				(3,588,000)		(23,974,000)
Purchases				10,002,000		9,423,000
Sales				(214,000)		(37,101,000)
Realized gains (losses), net				(3,106,000)		16,802,000
Unrealized gains (losses), net				1,958,000		(4,295,000)
Balance as of end of period				64,466,000		59,414,000
Level 3 investments in private equity investments owned by our consolidated venture capital fund, ownership percentage (in hundredths)				10.00%
Level 3 investments in private equity investments owned by our consolidated venture capital fund, non-controlling interest percentage (in hundredths)				90.00%
Transfers out of Level 3			3,600,000
Transfers out of Level 2	3,700,000	20,600,000
Fair Value, Measurements, Recurring [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets				340,548,000		323,104,000
U.S. Treasury bills				1,277,944,000		1,137,578,000
U.K. Treasury bills				119,000		7,911,000
Equity securities [Abstract]
Growth				107,991,000		97,418,000
Value				60,105,000		73,579,000
Blend				118,208,000		93,590,000
Other				45,583,000	[1]	33,919,000	[1]
Fixed Income securities [Abstract]
Taxable				124,550,000	[2]	151,613,000	[2]
Tax-exempt				16,109,000	[3]	10,060,000	[3]
Other				17,000		17,000
Derivatives				3,621,000		1,644,000
Long exchange-traded options				14,322,000		9,027,000
Private equity				76,058,000		107,588,000
Total assets measured at fair value				2,185,175,000		2,047,048,000
Securities sold not yet purchased [Abstract]
Short equities - corporate				34,469,000		42,914,000
Short exchange-traded options				3,567,000		7,622,000
Other				1,271,000		3,000
Derivatives				7,259,000		3,553,000
Total liabilities measured at fair value				46,566,000		54,092,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets				340,548,000		323,104,000
U.S. Treasury bills				0		0
U.K. Treasury bills				0		0
Equity securities [Abstract]
Growth				107,802,000		97,161,000
Value				60,096,000		73,579,000
Blend				118,208,000		93,590,000
Other				45,583,000	[1]	28,868,000	[1]
Fixed Income securities [Abstract]
Taxable				110,062,000	[2]	130,122,000	[2]
Tax-exempt				15,366,000	[3]	9,310,000	[3]
Other				17,000		17,000
Derivatives				127,000		16,000
Long exchange-traded options				14,322,000		9,027,000
Private equity				11,592,000		24,432,000
Total assets measured at fair value				823,723,000		789,226,000
Securities sold not yet purchased [Abstract]
Short equities - corporate				34,469,000		42,914,000
Short exchange-traded options				3,567,000		7,622,000
Other				1,271,000		3,000
Derivatives				2,054,000		318,000
Total liabilities measured at fair value				41,361,000		50,857,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets				0		0
U.S. Treasury bills				1,277,944,000		1,137,578,000
U.K. Treasury bills				119,000		7,911,000
Equity securities [Abstract]
Growth				189,000		188,000
Value				9,000		0
Blend				0		0
Other				0	[1]	5,051,000	[1]
Fixed Income securities [Abstract]
Taxable				14,488,000	[2]	21,491,000	[2]
Tax-exempt				743,000	[3]	750,000	[3]
Other				0		0
Derivatives				3,494,000		1,628,000
Long exchange-traded options				0		0
Private equity				0		23,811,000
Total assets measured at fair value				1,296,986,000		1,198,408,000
Securities sold not yet purchased [Abstract]
Short equities - corporate				0		0
Short exchange-traded options				0		0
Other				0		0
Derivatives				5,205,000		3,235,000
Total liabilities measured at fair value				5,205,000		3,235,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets				0		0
U.S. Treasury bills				0		0
U.K. Treasury bills				0		0
Equity securities [Abstract]
Growth				0		69,000
Value				0		0
Blend				0		0
Other				0	[1]	0	[1]
Fixed Income securities [Abstract]
Taxable				0	[2]	0	[2]
Tax-exempt				0	[3]	0	[3]
Other				0		0
Derivatives				0		0
Long exchange-traded options				0		0
Private equity				64,466,000		59,345,000
Total assets measured at fair value				64,466,000		59,414,000
Securities sold not yet purchased [Abstract]
Short equities - corporate				0		0
Short exchange-traded options				0		0
Other				0		0
Derivatives				0		0
Total liabilities measured at fair value				$ 0		$ 0

[1] Primarily long positions in corporate equities traded through our options desk.

[2] Primarily corporate and government securities.

[3] Primarily municipal bonds.