Fair Value - Narrative (Details) $ in Thousands	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	$ 79,250	$ 73,281
Revenue growth rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability	0.020		0.007
Revenue growth rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability	0.839		0.500
Revenue growth rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability	0.130		0.049
Discount rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability	0.019		0.019
Discount rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability	0.104		0.104
Discount rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability	0.066		0.080
Software publishing company
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	$ 2,200	2,100
Start-up company
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	300	300
Equity method investee
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	2,000	6,500
Broker dealer exchange memberships
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	$ 2,700	$ 3,300