Fair Value Measurements - Additional Information (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Long-term unsecured debt, carrying value	$ 74,048	$ 73,880
Long-term unsecured debt, Fair Value	69,740	70,314
ASU No. 2016-01
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Cumulative effect increase (net of taxes) in stockholders' equity	4,059
Private Equity Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Investment in equity securities and investment funds measured at fair value	$ 6,133	439
Investments in equity securities and investment funds, at carrying amount		1,675
Investments in equity securities and investment funds, at cost		1,236
Investments in equity securities and investment funds, fair value		$ 5,801
Private Equity Funds | Cost of Equity Discount Rate | Fair Value, Inputs, Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value discount rate	12
Private Equity Funds | Discount Factor for Lack of Marketability and Control | Fair Value, Inputs, Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value discount rate	8	20
Private Equity Funds | ASU No. 2016-01
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Cumulative effect increase (net of taxes) in stockholders' equity	$ 4,059