Fair Value (Details) (USD $)	3 Months Ended				9 Months Ended			9 Months Ended						9 Months Ended
	Sep. 30, 2012	Mar. 31, 2012	Sep. 30, 2011	Mar. 31, 2011	Sep. 30, 2012	Sep. 30, 2011	Jun. 30, 2011	Sep. 30, 2012 Private Equity Investment Venture Capital Fund [Member]	Sep. 30, 2012 Consolidated Private Equity Investment Venture Capital Fund [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member]		Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Minimum [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Maximum [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Private Equity Technology and Digital Media [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Private Equity Technology and Digital Media [Member] Minimum [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Private Equity Technology and Digital Media [Member] Maximum [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Private Equity Healthcare and Cleantech [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Private Equity Healthcare and Cleantech [Member] Minimum [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Private Equity Healthcare and Cleantech [Member] Maximum [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 1 [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Level 1 [Member]		Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 2 [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Level 2 [Member]		Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 3 [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets										$ 311,174,000		$ 340,548,000										$ 311,174,000		$ 340,548,000		$ 0		$ 0		$ 0		$ 0
U.S. Treasury bills										942,338,000		1,277,944,000										0		0		942,338,000		1,277,944,000		0		0
U.K. Treasury bills										124,000		119,000										0		0		124,000		119,000		0		0
Equity securities [Abstract]
Growth										114,538,000		107,991,000										114,538,000		107,802,000		0		189,000		0		0
Value										42,763,000		60,105,000										42,672,000		60,096,000		91,000		9,000		0		0
Blend										65,238,000		118,208,000										65,238,000		118,208,000		0		0		0		0
Other										23,776,000	[1]	45,583,000	[1]									23,765,000	[1]	45,583,000	[1]	11,000	[1]	0	[1]	0	[1]	0	[1]
Fixed Income securities [Abstract]
Taxable										178,263,000	[2]	124,550,000	[2]									177,744,000	[2]	110,062,000	[2]	519,000	[2]	14,488,000	[2]	0	[2]	0	[2]
Tax-exempt										6,385,000	[3]	16,109,000	[3]									5,602,000	[3]	15,366,000	[3]	783,000	[3]	743,000	[3]	0	[3]	0	[3]
Other												17,000												17,000				0				0
Derivatives										5,446,000		3,621,000										2,097,000		127,000		3,349,000		3,494,000		0		0
Long exchange-traded options										9,508,000		14,322,000										9,508,000		14,322,000		0		0		0		0
Private equity										72,361,000		76,058,000										10,699,000		11,592,000		0		0		61,662,000		64,466,000
Total assets measured at fair value										1,771,914,000		2,185,175,000										763,037,000		823,723,000		947,215,000		1,296,986,000		61,662,000		64,466,000
Securities sold not yet purchased [Abstract]
Short equities - corporate										18,830,000		34,469,000										18,830,000		34,469,000		0		0		0		0
Short exchange-traded options										4,860,000		3,567,000										4,860,000		3,567,000		0		0		0		0
Other										1,681,000		1,271,000										1,681,000		1,271,000		0		0		0		0
Derivatives										6,691,000		7,259,000										32,000		2,054,000		6,659,000		5,205,000		0		0
Total liabilities measured at fair value										32,062,000		46,566,000										25,403,000		41,361,000		6,659,000		5,205,000		0		0
Transfers out of Level 3		13,500,000		3,600,000
Transfers out of Level 2	6,000,000		3,700,000		6,000,000	3,700,000	20,600,000
Fair value measurement Level 3 reconciliation [Abstract]
Balance as of beginning of period	57,908,000	64,466,000	59,524,000	59,414,000	64,466,000	59,414,000
Transfers (out) in, net	0		0		(13,548,000)	(3,588,000)
Purchases	3,467,000		577,000		7,300,000	7,993,000
Sales	0		(1,000)		(1,823,000)	(214,000)
Realized gains (losses), net	(8,379,000)		(246,000)		(7,524,000)	(3,106,000)
Unrealized gains (losses), net	8,666,000		(1,371,000)		12,791,000	(2,016,000)
Balance as of end of period	61,662,000		58,483,000		61,662,000	58,483,000
Percentage of consolidated venture capital fund the Company owned (in hundredths)	10.00%				10.00%
Percentage of consolidated venture capital fund non-controlling interests owned (in hundredths)	90.00%				90.00%
Quantitative information about Level 3 fair value measurements [Abstract]
Fair value																23,256,000			14,871,000
Valuation technique																Market comparable companies			Market comparable companies
Revenue multiple																	2.5	6		0.7	45.9
Discount rate (in hundredths)																18.00%
Discount years - minimum																1
Discount years - maximum																2
R & D Multiple - minimum																			1.3
R & D Multiple - maximum																			37.4
Discount for lack of marketability and risk factors (in hundredths)														50.00%	60.00%
Description of investment								Communications, consumer, digital media, healthcare and information technology markets	Clean energy, resource and energy efficiency and other sustainable industries
Private equity investment at fair value								22,300,000	1,200,000
Unfunded commitments to investments								$ 16,800,000	$ 200,000

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

[2] Primarily corporate and government securities.

[3] Primarily municipal bonds.