Income Taxes - Tax Receivable Agreement (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Jun. 30, 2017
Tax Receivable Agreement
Cash savings percentage (as a percent)	85.00%
Benefits of cash savings retained under tax receivable agreement (as a percent)	15.00%
Gross liability under tax receivable agreement	$ 60,700	$ 45,700
Increase (decrease) in TRA liability due to valuation allowance recorded	(4,900)	(2,700)
Liability under tax receivable agreement	55,096	43,045	$ 14,140
Valuation allowance	65,700	$ 50,600
Anticipated payment	$ 600