Stockholders Equity - Additional Information (Details) $ in Thousands	1 Months Ended						9 Months Ended		48 Months Ended
	Aug. 31, 2018 shares	Mar. 31, 2018 shares	Jul. 31, 2017 shares	Jan. 31, 2017 shares	Nov. 30, 2014 shares	Apr. 30, 2014 shares	Sep. 30, 2018 USD ($) shares	Sep. 30, 2017 USD ($) shares	Sep. 30, 2018 shares	Dec. 31, 2017 shares
Class Of Stock [Line Items]
Treasury stock shares acquired (in shares)							484,061	264,710
Treasury stock shares acquired | $							$ 24,692	$ 9,832
Number of units held by noncontrolling interest holders							13,053,465		13,053,465	22,472,337
Group LP
Class Of Stock [Line Items]
Noncontrolling interests (as a percent)							22.00%		22.00%	40.00%
Class A Common Stock
Class Of Stock [Line Items]
Aggregate stock issuance (in shares)						15,263,653
Common stock, shares issued							46,908,723		46,908,723	34,163,042
Common stock, shares outstanding							45,717,246		45,717,246	33,455,626
Total Increase in Shares Outstanding	4,685,217	4,689,295	5,680,903	5,356,876	4,511,058				24,923,349
Number of shares of common stock to be issued upon exchange of a partnership unit							1		1
Class A Common Stock | Group LP
Class Of Stock [Line Items]
Common stock, shares outstanding							45,717,246		45,717,246	33,455,626
Class B Common Stock
Class Of Stock [Line Items]
Common stock, shares issued							10,493,358		10,493,358	19,912,230
Common stock, shares outstanding							10,493,358		10,493,358	19,912,230
Total Increase in Shares Outstanding						36,158,698
Ratio of subscription price to the initial public offering price of shares of common stock					0.00055
Dividends payable ratio to outstanding shares of publicly traded common stock					0.00055