Fair Value - Additional Information (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) acquisition	Dec. 31, 2016 USD ($) acquisition $ / shares	Dec. 31, 2015 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of private equity investment	$ 37,200,000
Contingent payment arrangements	0	$ 11,893,000	$ 0
Changes in estimates	(193,000)	(21,482,000)
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of private equity investment	37,326,000	40,413,000
Contingent payment arrangements	10,855,000	17,589,000
Private Equity Investment Venture Capital Fund [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of investment	$ 1,000,000	$ 4,900,000
Contingent Payment Arrangements [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of businesses acquired | acquisition	1	3
Contingent Payment Arrangements [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent payment arrangements	$ 10,900,000	$ 17,600,000
Contingent Payment Arrangements [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
AUM growth rates weighted average		18.00%
Contingent Payment Arrangements [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Revenue growth rate		4.00%
Discount rate	1.40%	1.40%
Contingent Payment Arrangements [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Revenue growth rate	31.00%	31.00%
Discount rate	2.30%	6.40%
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of investment	$ 1,071,000	$ 5,023,000
Contingent payment arrangements	$ 10,855,000	17,589,000
2010 Acquisition [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Changes in estimates		2,200,000
2013 Acquisition [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Changes in estimates		$ 19,300,000
Additional amount per share shareholders are entitled to if the assets under management exceed the threshold (in dollars per unit) | $ / shares		$ 4
Assets under management threshold		$ 5,000,000,000