Fair Value (Details) (USD $)	3 Months Ended		6 Months Ended										6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Median [Member]	Jun. 30, 2013 Private Equity Investment Venture Capital Fund [Member]	Jun. 30, 2013 Private Equity Fund [Member]	Jun. 30, 2013 Consolidated Private Equity Investment Venture Capital Fund [Member]	Jun. 30, 2013 Fair Value, Measurements, Recurring [Member]		Dec. 31, 2012 Fair Value, Measurements, Recurring [Member]		Jun. 30, 2013 Fair Value, Measurements, Recurring [Member] Private Equity Investment Venture Capital Fund [Member]	Jun. 30, 2013 Fair Value, Measurements, Recurring [Member] Private Equity Investment Venture Capital Fund [Member] Minimum [Member]	Jun. 30, 2013 Fair Value, Measurements, Recurring [Member] Private Equity Investment Venture Capital Fund [Member] Maximum [Member]	Jun. 30, 2013 Fair Value, Measurements, Recurring [Member] Private Equity Healthcare and Cleantech [Member]	Jun. 30, 2013 Fair Value, Measurements, Recurring [Member] Private Equity Healthcare and Cleantech [Member] Minimum [Member]		Jun. 30, 2013 Fair Value, Measurements, Recurring [Member] Private Equity Healthcare and Cleantech [Member] Maximum [Member]		Jun. 30, 2013 Fair Value, Measurements, Recurring [Member] Level 1 [Member]		Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Level 1 [Member]		Jun. 30, 2013 Fair Value, Measurements, Recurring [Member] Level 2 [Member]		Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Level 2 [Member]		Jun. 30, 2013 Fair Value, Measurements, Recurring [Member] Level 3 [Member]		Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets									$ 275,458,000		$ 170,120,000										$ 275,458,000		$ 170,120,000		$ 0		$ 0		$ 0		$ 0
U.S. Treasury bills									824,061,000		1,537,150,000										0		0		824,061,000		1,537,150,000		0		0
U.K. Treasury bills									117,000		125,000										0		0		117,000		125,000		0		0
Equity securities [Abstract]
Growth									116,486,000		125,242,000										116,486,000		125,242,000		0		0		0		0
Value									36,356,000		36,126,000										36,356,000		36,126,000		0		0		0		0
Multi-asset and asset allocation									50,195,000		59,449,000										50,195,000		59,449,000		0		0		0		0
Other									56,399,000	[1]	39,702,000	[1]									56,399,000	[1]	39,702,000	[1]	0	[1]	0	[1]	0	[1]	0	[1]
Fixed Income securities [Abstract]
Taxable									149,698,000	[2]	178,854,000	[2]									146,730,000	[2]	177,635,000	[2]	2,968,000	[2]	1,219,000	[2]	0	[2]	0	[2]
Tax-exempt									6,022,000	[3]	6,458,000	[3]									5,227,000	[3]	5,661,000	[3]	795,000	[3]	797,000	[3]	0	[3]	0	[3]
Derivatives									7,626,000		2,625,000										1,619,000		64,000		6,007,000		2,561,000		0		0
Long exchange-traded options									13,074,000		15,087,000										13,074,000		15,087,000		0		0		0		0
Private equity									85,876,000		84,648,000										5,229,000		7,695,000		19,221,000		0		61,426,000		76,953,000
Total assets measured at fair value									1,621,368,000		2,255,586,000										706,773,000		636,781,000		853,169,000		1,541,852,000		61,426,000		76,953,000
Securities sold not yet purchased [Abstract]
Short equities - corporate									37,769,000		54,370,000										37,769,000		54,370,000		0		0		0		0
Short exchange-traded options									15,065,000		9,197,000										15,065,000		9,197,000		0		0		0		0
Other											271,000												271,000				0				0
Derivatives									1,278,000		7,070,000										70,000		1,598,000		1,208,000		5,472,000		0		0
Total liabilities measured at fair value									54,112,000		70,908,000										52,904,000		65,436,000		1,208,000		5,472,000		0		0
Transfers out of Level 3	19,200,000	13,500,000	19,200,000
Fair value measurement Level 3 reconciliation [Abstract]
Balance as of beginning of period	74,853,000	58,235,000	76,953,000	64,466,000
Transfers (out) in, net	(19,220,000)	0	(19,220,000)	(13,548,000)
Purchases	87,000	1,988,000	97,000	3,833,000
Sales	(34,000)	(1,823,000)	(34,000)	(1,823,000)
Realized gains (losses), net	(6,581,000)	855,000	(6,581,000)	855,000
Unrealized gains (losses), net	12,321,000	(1,347,000)	10,211,000	4,125,000
Balance as of end of period	61,426,000	57,908,000	61,426,000	57,908,000
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			1,849,000
Valuation technique													Market comparable companies			Market comparable companies
Revenue multiple					8.4									2.5	6.0		0.7	[4]	62.5	[4]
Discount rate (in hundredths)													18.00%
Discount years													1.5
R & D Multiple					5.2												1.0	[4]	32.4	[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	energy sector	clean energy, resource and energy efficiency and other sustainable industries
Private equity investment at fair value						28,500,000	7,700,000	200,000
Unfunded commitments to investments						$ 12,200,000	$ 100,000	$ 200,000
Number of securities going public	1		1	1

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

[2] Primarily corporate and government securities.

[3] Primarily municipal bonds.

[4] The median for the Healthcare and Cleantech revenue multiple is 8.4; the median R&D multiple is 5.2.