Income Per Share (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Document Period End Date			Sep. 30, 2018
Income per share:
Shares used to calculate basic income per share (in shares)	30,185	30,749	30,230	30,739	[1]
Dilutive effect of outstanding stock options, performance stock units and restricted stock units (in shares)	248	326	270	359
Shares used to calculate diluted income per share (in shares)	30,433	31,075	30,500	31,098	[1]
Stock options
Income per share:
Anti-dilutive securities excluded from computation of diluted income per share (in shares)	9	7	10	8
Restricted stock units
Income per share:
Anti-dilutive securities excluded from computation of diluted income per share (in shares)	1	0	0	0

[1] See Note 2.