Stockholders' Equity - Number of Shares (Details) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 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	36,891	37,088
Effect of using weighted average common shares outstanding	32	104
Shares used in computing earnings per share-basic	36,923	37,192
Effect of shares issuable under stock option plans based on the treasury stock method	230	317
Effect of restricted and contingently issuable shares	81	119
Shares used in computing earnings per share-diluted	37,234	37,628