Fair Value - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) acquisition	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Changes in estimates	$ (2,429)	$ (193)
Investments, measurements other than fair value	96,840	54,229
Contingent payment arrangements	0	0	$ 11,893
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent payment arrangements	$ 7,336	10,855
Private equity investment venture capital fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of investment		1,000
Contingent payment arrangements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of businesses acquired | acquisition	1
Contingent payment arrangements | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent payment arrangements	$ 7,300	10,900
Private equity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of private equity investment		37,200
Start-up company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, measurements other than fair value	900	4,600
Equity method investee
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, measurements other than fair value	3,400	4,100
Broker dealer exchange memberships
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, measurements other than fair value	$ 3,100	$ 3,200
Revenue growth rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration liability, measurement input	0.18
Revenue growth rate | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration liability, measurement input		0.31
Discount rate | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration liability, measurement input	0.032	0.014
Discount rate | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration liability, measurement input	0.037	0.023
2016 Acquisition
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Changes in estimates	$ 2,400