Fair Value (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($) acquisition
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investments, NAV Expedient	$ 37,600
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
Contingent Payment Arrangements
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Number of acquisitions | acquisition		3
Private Equity Investment Energy Fund
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investments	1,000	$ 4,900
Recurring
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investments, NAV Expedient	37,710	40,413
Contingent payment arrangements	12,103	17,589
Recurring | Contingent Payment Arrangements
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Contingent payment arrangements	$ 12,100	$ 17,600
Weighted Average | Contingent Payment Arrangements
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
AUM growth rate		18.00%
Minimum | Contingent Payment Arrangements
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Revenue growth rate		4.00%
Discount rate	1.40%	1.40%
Maximum | Contingent Payment Arrangements
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Revenue growth rate	31.00%	31.00%
Discount rate	2.30%	6.40%
Private equity
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unfunded commitment	$ 800