Fair Value (Details) (USD $)	3 Months Ended										3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Median [Member]	Mar. 31, 2013 Private Equity Investment Venture Capital Fund [Member]	Mar. 31, 2013 Private Equity Fund [Member]	Mar. 31, 2013 Consolidated Private Equity Investment Venture Capital Fund [Member]	Mar. 31, 2013 Fair Value, Measurements, Recurring [Member]		Dec. 31, 2012 Fair Value, Measurements, Recurring [Member]		Mar. 31, 2013 Fair Value, Measurements, Recurring [Member] Private Equity Investment Venture Capital Fund [Member]	Mar. 31, 2013 Fair Value, Measurements, Recurring [Member] Private Equity Investment Venture Capital Fund [Member] Minimum [Member]	Mar. 31, 2013 Fair Value, Measurements, Recurring [Member] Private Equity Investment Venture Capital Fund [Member] Maximum [Member]	Mar. 31, 2013 Fair Value, Measurements, Recurring [Member] Private Equity Healthcare and Cleantech [Member]	Mar. 31, 2013 Fair Value, Measurements, Recurring [Member] Private Equity Healthcare and Cleantech [Member] Minimum [Member]		Mar. 31, 2013 Fair Value, Measurements, Recurring [Member] Private Equity Healthcare and Cleantech [Member] Maximum [Member]		Mar. 31, 2013 Fair Value, Measurements, Recurring [Member] Level 1 [Member]		Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Level 1 [Member]		Mar. 31, 2013 Fair Value, Measurements, Recurring [Member] Level 2 [Member]		Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Level 2 [Member]		Mar. 31, 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							$ 216,435,000		$ 170,120,000										$ 216,435,000		$ 170,120,000		$ 0		$ 0		$ 0		$ 0
U.S. Treasury bills							1,082,237,000		1,537,150,000										0		0		1,082,237,000		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							141,695,000		125,242,000										141,695,000		125,242,000		0		0		0		0
Value							36,144,000		36,126,000										32,440,000		36,126,000		3,704,000		0		0		0
Multi-asset and asset allocation							55,981,000		59,449,000										55,981,000		59,449,000		0		0		0		0
Other							56,137,000	[1]	39,702,000	[1]									56,137,000	[1]	39,702,000	[1]	0	[1]	0	[1]	0	[1]	0	[1]
Fixed Income securities [Abstract]
Taxable							150,016,000	[2]	178,854,000	[2]									147,024,000	[2]	177,635,000	[2]	2,992,000	[2]	1,219,000	[2]	0	[2]	0	[2]
Tax-exempt							6,492,000	[3]	6,458,000	[3]									5,701,000	[3]	5,661,000	[3]	791,000	[3]	797,000	[3]	0	[3]	0	[3]
Derivatives							2,931,000		2,625,000										821,000		64,000		2,110,000		2,561,000		0		0
Long exchange-traded options							13,772,000		15,087,000										13,772,000		15,087,000		0		0		0		0
Private equity							80,669,000		84,648,000										5,816,000		7,695,000		0		0		74,853,000		76,953,000
Total assets measured at fair value							1,842,626,000		2,255,586,000										675,822,000		636,781,000		1,091,951,000		1,541,852,000		74,853,000		76,953,000
Securities sold not yet purchased [Abstract]
Short equities - corporate							57,588,000		54,370,000										57,588,000		54,370,000		0		0		0		0
Short exchange-traded options							11,845,000		9,197,000										11,845,000		9,197,000		0		0		0		0
Other									271,000												271,000				0				0
Derivatives							4,081,000		7,070,000										1,336,000		1,598,000		2,745,000		5,472,000		0		0
Total liabilities measured at fair value							73,514,000		70,908,000										70,769,000		65,436,000		2,745,000		5,472,000		0		0
Transfers out of Level 3		13,500,000
Fair value measurement Level 3 reconciliation [Abstract]
Balance as of beginning of period	76,953,000	64,466,000
Transfers (out) in, net	0	(13,548,000)
Purchases	10,000	1,845,000
Sales	0	0
Realized gains (losses), net	0	0
Unrealized gains (losses), net	(2,110,000)	5,472,000
Balance as of end of period	74,853,000	58,235,000
Percentage of consolidated venture capital fund the Company owned (in hundredths)	10.00%
Percentage of consolidated venture capital fund non-controlling interests owned (in hundredths)	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			7.8									2.5	6		0.6	[4]	68.7	[4]
Discount rate (in hundredths)											18.00%
Discount years											1.75
R & D Multiple			5.2												0.9	[4]	35.0	[4]
Discount for lack of marketability and risk factors (in hundredths)															30.00%		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				28,700,000	7,700,000	300,000
Unfunded commitments to investments				$ 12,200,000	$ 200,000	$ 200,000

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

[2] Primarily corporate and government securities.

[3] Primarily municipal bonds.

[4] The median for the Healthcare and Cleantech revenue multiple is 7.8; the median R&D multiple is 5.2.