Stockholders' Equity - Number of Shares (Details) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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	37,088	37,226
Effect of using weighted average common shares outstanding	104	(1)
Shares used in computing earnings per share—basic	37,192	37,225
Effect of shares issuable under stock option plans based on the treasury stock method	317	335
Effect of restricted and contingently issuable shares	119	164
Shares used in computing earnings per share—diluted	37,628	37,724