Stockholders' Equity - Number of Shares (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 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	37,221	37,134	37,221	37,134
Effect of using weighted average common shares outstanding	73	98	15	125
Shares used in computing earnings per share—basic	37,294	37,232	37,236	37,259
Effect of shares issuable under stock option plans based on the treasury stock method	280	293	294	312
Effect of restricted and contingently issuable shares	93	101	104	113
Shares used in computing earnings per share—diluted	37,667	37,626	37,634	37,684