Compensation Plans Employee Stock Purchase Plan (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Compensation plans
Number of shares available for grant	3,600,000
Proceeds from sale of common stock	$ 4,877,000	$ 6,858,000	$ 5,816,000
Employee Stock [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	1,100,000
Issuance of common stock under Employee Stock Purchase Plan (in shares)	83,136	77,427	66,485
Proceeds from sale of common stock	4,100,000	3,700,000	2,800,000
Risk free interest rate	0.10%	0.10%	0.10%
Dividend yield	0.10%	0.20%	0.20%
Volatility factor of the expected market price of the Company's common stock	33.00%	41.10%	47.80%
Expected life (in years)	6 months	6 months	6 months
Stock-based compensation expense	$ 1,100,000	$ 1,100,000	$ 948,000