Fair Value (Details) $ in Thousands		3 Months Ended	9 Months Ended
	Apr. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments		$ 96,840	$ 86,317
Transfer of securities from level 2 to level 1			3,200
Contingent payment arrangements			17,384	$ 0
Private Equity Investment Energy Fund
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments					$ 1,000
Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent payment arrangements		7,336	26,432
Liabilities measured at fair value for impairment on a nonrecurring basis		30,781	56,780
Recurring | Contingent Payment Arrangements
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent payment arrangements		7,300	26,400
Software publishing company
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments			1,000
Start-up company
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments		900	900
Equity method investee
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments		3,400	2,500
Broker dealer exchange memberships
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments		3,100	$ 3,300
2016 Acquisition
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration written off		$ 2,400
2016 Acquisition | Revenue growth rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability			0.26
2016 Acquisition | Discount rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability			0.032
2016 Acquisition | Discount rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability			0.037
Autonomous
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent payment arrangements	$ 17,400
Contingent consideration payable, term	5 years		5 years
Autonomous | Revenue growth rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability			0.007
Autonomous | Revenue growth rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability			0.025
Autonomous | Risk-free rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability			0.035
Autonomous | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability			0.104
Autonomous | Market price of risk adjustment rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability			0.069