Fair Value (Details)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	$ 135,713,000	$ 54,229,000
Private Equity Investment Energy Fund
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments		1,000,000
Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent payment arrangements	11,013,000	10,855,000
Liabilities measured at fair value for impairment on a nonrecurring basis	45,911,000	55,139,000
Recurring | Contingent Payment Arrangements
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent payment arrangements	11,000,000	10,900,000
Nonrecurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets measured at fair value for impairment on a nonrecurring basis	0	0
Liabilities measured at fair value for impairment on a nonrecurring basis	0	0
Private equity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, NAV Expedient	36,400,000
Unfunded commitment	0
Start-up company
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	3,600,000	4,600,000
Equity method investee
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	3,600,000	4,100,000
Broker dealer exchange memberships
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other investments	$ 3,100,000	$ 3,200,000
Revenue growth rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability	0.31
Discount rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability	0.014
Discount rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition-related contingent liability	0.023