Income Per Share (Details) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income per share:
Shares used to calculate basic income per share (in shares)	30,193	30,755	30,253	30,734
Dilutive effect of outstanding stock options, performance stock units and restricted stock units (in shares)	283	340	280	376
Shares used to calculate diluted income per share (in shares)	30,476	31,095	30,533	31,110
Stock options
Income per share:
Anti-dilutive securities excluded from computation of diluted income per share (in shares)	5	7	11	8
Restricted stock units
Income per share:
Anti-dilutive securities excluded from computation of diluted income per share (in shares)	0	0	2	0