Compensation Plans Employee Stock Purchase Plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compensation plans
Number of shares available for grant	3,300,000
Employee Stock Purchase Plan [Member]
Compensation plans
Maximum employee subscription rate	15.00%
Share-based Compensation Arrangement by Share-based Payment Award, Maximum Employee Subscription	$ 25,000
Percent of offering date price paid	85.00%
Number of shares available for grant	1,800,000
Issuance of common stock under Employee Stock Purchase Plan (in shares)	186,665	218,135	197,214
Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Excluding Stock Options	$ 2,600,000	$ 4,200,000	$ 4,800,000
Risk free interest rate	0.90%	0.40%	0.10%
Dividend yield	0.50%	0.40%	0.20%
Volatility factor of the expected market price of the Company's common stock	62.50%	95.00%	61.20%
Expected life (in years)	6 months	6 months	6 months
Stock-based compensation expense	$ 1,000,000	$ 2,000,000	$ 1,800,000