Stockholders' Equity - Number of Shares (Details) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reconciliation of the number of shares outstanding with the number of shares used in computing basic and diluted earnings per share
Common shares outstanding, end of period	37,246	37,288	37,246	37,288
Effect of using weighted average common shares outstanding	(26)	8	(40)	(16)
Shares used in computing earnings per share—basic	37,220	37,296	37,206	37,272
Effect of shares issuable under stock option plans based on the treasury stock method	309	308	341	321
Effect of restricted and contingently issuable shares	76	101	70	121
Shares used in computing earnings per share—diluted	37,605	37,705	37,617	37,714