Stock-Based Compensation (Details) - Weighted-Average Assumptions Used To Determine Fair Value Of SARs Grants On Date Of Grant Using Black-Scholes-Merton Valuation Model - Stock Appreciation Rights (SARs)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%	0.00%
Expected stock price volatility	22.00%	22.00%	24.00%
Risk-free interest rate	1.80%	1.10%	1.50%
Expected life in years	4 years 6 months 11 days	4 years 4 months 21 days	4 years 4 months 28 days