Stockholders' Equity - Number of Shares (Details) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
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	36,854	37,246	36,854	37,246
Effect of using weighted average common shares outstanding	89	(26)	79	(40)
Shares used in computing earnings per share-basic	36,943	37,220	36,933	37,206
Effect of shares issuable under stock option plans based on the treasury stock method	221	309	226	341
Effect of restricted and contingently issuable shares	59	76	69	70
Shares used in computing earnings per share-diluted	37,223	37,605	37,228	37,617