Fair Value of Financial Instruments - Investments at Fair Value Using Net Asset Value Per Share As Practical Expedient (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Limited liability investments, at fair value	$ 33,047	$ 26,015
Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Limited liability investments, at fair value	$ 22,114	$ 21,685