Equity-Based Compensation - Stock Options (Details) - Stock options - USD ($) $ / shares in Units, $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Stock options
Outstanding at the beginning of the period (in shares)	660,512
Options forfeited (in shares)	(51,582)
Outstanding at the end of the period (in shares)	608,930	660,512
Vested or expected to vest (in shares)	608,930
Exercisable (in shares)	481,842
Weighted average exercise price
Outstanding at the beginning of the period (in dollars per share)	$ 50.48
Options forfeited (in dollars per share)	50.00
Outstanding at the end of the period (in dollars per share)	50.52	$ 50.48
Vested or expected to vest (in dollars per share)	50.52
Exercisable (in dollars per share)	$ 50.66
Weighted average remaining contractual life
Outstanding	6 years 1 month 24 days	7 years 22 days
Vested or expected to vest	6 years 1 month 24 days
Exercisable	6 years 18 days
Additional disclosures
Unrecognized stock-based compensation expense	$ 1.0
Weighted average period for recognizing unrecognized stock-based compensation expense	6 months
Maximum
Stock-based compensation
Contractual life	10 years