Fair Value (Details) $ in Thousands	3 Months Ended
	Dec. 31, 2018 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	$ 96,840	$ 93,092
Transfer of securities from level 2 to level 1		3,200
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	7,390
Liabilities measured at fair value for impairment on a nonrecurring basis	30,781	27,123
Recurring | Contingent Payment Arrangements
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent payment arrangements	7,300	7,400
Start-up company
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	900
Equity method investee
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	3,400	2,900
Broker dealer exchange memberships
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	3,100	$ 3,300
Revenue growth rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability		0.18
Discount rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability		0.032
Discount rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability		0.037
2016 Acquisition
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration written off	$ 2,400