Share-Based Compensation - Performance Shares Assumptions (Details) - Performance shares - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assumptions:
Weighted average grant date fair value (in dollars per share)	$ 39.42	$ 21.10	$ 30.63
Expected volatility	32.10%	34.02%	42.70%
Risk-free rate	1.39%	0.86%	1.09%
Expected term (in years)	2 years 11 months 14 days	2 years 11 months 13 days	2 years 11 months 12 days
Expected dividends	$ 0	$ 0	$ 0