Segments - Summary of Certain Financial Data For Each of Segments (Parenthetical) (Details) - USD ($) $ in Thousands	9 Months Ended			12 Months Ended
	Sep. 30, 2017	[1]	Sep. 30, 2016	Sep. 30, 2018		Dec. 31, 2016	[1]
Segment Reporting Information [Line Items]
Equity-based compensation expense	$ 3,839		$ 3,718	$ 28,795	[1]	$ 2,772
IPO
Segment Reporting Information [Line Items]
Equity-based compensation expense				$ 19,590

[1] Represents equity-based compensation expense recognized for equity incentive plans outstanding, including $19,590 related to the IPO in the fiscal year ended September 30, 2018.