Stock-Based Compensation Stock Option Black Scholes Option Pricing Model Assumptions (Details) - Employee Stock Option [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Expected volatility	32.50%	33.50%	34.30%
Risk-free interest rate	2.00%	1.30%	1.70%
Expected term (years)	5 years 6 months	5 years 6 months	5 years 6 months
Granted, Weighted Average Grant Date Fair Value	$ 21.99	$ 13.27	$ 12.09