Income Per Share (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income per share:
Basic weighted average shares outstanding	30,671	31,032	31,288
Dilutive potential common shares	339	269	301
Diluted weighted average shares outstanding	31,010	31,301	31,589
Stock options
Income per share:
Anti-dilutive securities excluded from computation of diluted income per share	8	8	12