Stock Based Compensation Fair Value Assumptions for Stock Options (Details) - Five Year Service Stock Options [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Risk-free interest rate	1.96%	1.34%	1.72%
Expected volatility	39.49%	47.20%	56.37%
Expected term (in years)	5 years 10 months 20 days	5 years 8 months 19 days	5 years 7 months 6 days
Weighted average grant date fair value of options granted during the period	$ 9.45	$ 7.57	$ 11.07