Fair Value (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investments, NAV Expedient	$ 37,100,000
Other investments	56,058,000	$ 54,229,000
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 Investment Energy Fund
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investments	1,000,000	1,000,000
Recurring
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investments, NAV Expedient	37,470,000	37,326,000
Contingent payment arrangements	10,908,000	10,855,000
Recurring | Contingent Payment Arrangements
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Contingent payment arrangements	$ 10,900,000	10,900,000
Minimum | Contingent Payment Arrangements
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Revenue growth rate	31.00%
Discount rate	1.40%
Maximum | Contingent Payment Arrangements
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	2.30%
Private equity
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unfunded commitment	$ 0
Start-up company
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other investments	4,600,000	4,600,000
Equity method investee
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other investments	4,000,000	4,100,000
Broker dealer exchange memberships
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other investments	$ 3,200,000	$ 3,200,000