Fair Value (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015 Acquisition	Mar. 31, 2014	Dec. 31, 2014
Fair value measurement Level 3 reconciliation [Abstract]
Balance as of beginning of period	$ 58,926,000	$ 52,081,000	$ 52,081,000
Transfers out	(26,000)	0
Purchases	122,000	501,000
Sales	0	(1,121,000)
Realized (losses) gains, net	(5,194,000)	1,121,000
Unrealized gains (losses), net	5,013,000	(1,838,000)
Balance as of end of period	58,841,000	50,744,000
Percentage of consolidated venture capital fund non-controlling interests owned (in hundredths)	90.00%
Quantitative information about Level 3 fair value measurements [Abstract]
Number of acquisition	3
Fair Value, Liabilities Measurement Level 3 reconciliation
Balance as of beginning of period	42,436,000	38,205,000	38,205,000
Accretion	443,000	321,000
Payments	(656,000)	(838,000)
Balance as of end of period	42,223,000	37,688,000
Minimum [Member]
Quantitative information about Level 3 fair value measurements [Abstract]
Discount rate (in hundredths)	3.00%
Maximum [Member]
Quantitative information about Level 3 fair value measurements [Abstract]
Discount rate (in hundredths)	18.00%
Weighted Average [Member]
Quantitative information about Level 3 fair value measurements [Abstract]
AUM growth rate (in hundredths)	46.00%
Revenue growth rate (in hundredths)	71.00%
Private Equity Technology, Media and Telecommunications [Member]
Quantitative information about Level 3 fair value measurements [Abstract]
Description of investment	communications, consumer, digital media, healthcare and information technology markets
Private equity investment at fair value	31,000,000
Unfunded commitments to investments	2,900,000
Private Equity Healthcare and Cleantech [Member]
Quantitative information about Level 3 fair value measurements [Abstract]
Number of investment	2		2
Fair value of investments	100,000		200,000
Private Equity Fund [Member]
Quantitative information about Level 3 fair value measurements [Abstract]
Fair value of investments	7,500,000		7,500,000
Description of investment	energy sector
Consolidated Private Equity Investment Venture Capital Fund [Member]
Quantitative information about Level 3 fair value measurements [Abstract]
Description of investment	clean energy, resource and energy efficiency and other sustainable industries
Private equity investment at fair value	100,000
Unfunded commitments to investments	0
Consolidated Private Equity Fund [Member]
Securities sold not yet purchased [Abstract]
Percentage of consolidated venture capital fund the Company owned (in hundredths)	10.00%		10.00%
Fair Value, Measurements, Recurring [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets	172,415,000		89,566,000
U.S. Treasury bills	499,059,000		444,152,000
Available-for-sale [Abstract]
Equity securities	6,136,000		5,951,000
Fixed income securities	306,000		221,000
Trading [Abstract]
Equity securities	359,594,000		387,502,000
Fixed income securities	186,313,000		167,059,000
Long exchange-traded options	6,767,000		22,290,000
Derivatives	8,551,000		6,787,000
Private equity	72,604,000		71,088,000
Total assets measured at fair value	1,311,745,000		1,194,616,000
Securities sold not yet purchased [Abstract]
Short equities - corporate	61,302,000		81,784,000
Short exchange-traded options	5,602,000		7,118,000
Derivatives	9,773,000		9,392,000
Contingent payment arrangements	42,223,000		42,436,000
Total liabilities measured at fair value	118,900,000		140,730,000
Fair Value, Measurements, Recurring [Member] | Private Equity Technology, Media and Telecommunications [Member]
Quantitative information about Level 3 fair value measurements [Abstract]
Fair value	20,044,000		20,112,000
Valuation technique	Market comparable companies		Market comparable companies
Marketability discount (in hundredths)	30.00%
Discount rate (in hundredths)			18.00%
Discount years			2 years
Fair Value, Measurements, Recurring [Member] | Private Equity Technology, Media and Telecommunications [Member] | Minimum [Member]
Quantitative information about Level 3 fair value measurements [Abstract]
Revenue multiple	2.10		2.00
Fair Value, Measurements, Recurring [Member] | Private Equity Technology, Media and Telecommunications [Member] | Maximum [Member]
Quantitative information about Level 3 fair value measurements [Abstract]
Revenue multiple	4.60		3.50
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets	172,415,000		89,566,000
U.S. Treasury bills	0		0
Available-for-sale [Abstract]
Equity securities	6,136,000		5,951,000
Fixed income securities	306,000		221,000
Trading [Abstract]
Equity securities	334,792,000		387,495,000
Fixed income securities	183,227,000		164,317,000
Long exchange-traded options	6,767,000		22,290,000
Derivatives	360,000		571,000
Private equity	13,885,000		12,162,000
Total assets measured at fair value	717,888,000		682,573,000
Securities sold not yet purchased [Abstract]
Short equities - corporate	61,302,000		81,784,000
Short exchange-traded options	5,602,000		7,118,000
Derivatives	3,011,000		2,438,000
Contingent payment arrangements	0		0
Total liabilities measured at fair value	69,915,000		91,340,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets	0		0
U.S. Treasury bills	499,059,000		444,152,000
Available-for-sale [Abstract]
Equity securities	0		0
Fixed income securities	0		0
Trading [Abstract]
Equity securities	24,680,000		7,000
Fixed income securities	3,086,000		2,742,000
Long exchange-traded options	0		0
Derivatives	8,191,000		6,216,000
Private equity	0		0
Total assets measured at fair value	535,016,000		453,117,000
Securities sold not yet purchased [Abstract]
Short equities - corporate	0		0
Short exchange-traded options	0		0
Derivatives	6,762,000		6,954,000
Contingent payment arrangements	0		0
Total liabilities measured at fair value	6,762,000		6,954,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets	0		0
U.S. Treasury bills	0		0
Available-for-sale [Abstract]
Equity securities	0		0
Fixed income securities	0		0
Trading [Abstract]
Equity securities	122,000		0
Fixed income securities	0		0
Long exchange-traded options	0		0
Derivatives	0		0
Private equity	58,719,000		58,926,000
Total assets measured at fair value	58,841,000		58,926,000
Securities sold not yet purchased [Abstract]
Short equities - corporate	0		0
Short exchange-traded options	0		0
Derivatives	0		0
Contingent payment arrangements	42,223,000		42,436,000
Total liabilities measured at fair value	$ 42,223,000		$ 42,436,000