Equity-Based Compensation - Weighted Average Assumptions Used in Estimating Grant Date Fair Value of Stock Options (Details) - Incentive Units	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted Average Assumptions:
Expected volatility	29.30%	39.80%	35.10%
Risk-free interest rate	1.40%	1.50%	1.40%
Expected term	2 years	3 years 8 months 12 days	3 years 1 month 6 days
Dividend rate	0.60%	0.90%	1.30%