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Stockholders' Equity - Number of Shares (Details) - shares
shares in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
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 37,187 37,209 37,427
Effect of using weighted average common shares outstanding 52 126 15
Shares used in computing earnings per share—basic 37,239 37,335 37,442
Effect of shares issuable under stock option plans based on the treasury stock method 316 330 266
Effect of restricted and contingently issuable shares 117 146 160
Shares used in computing earnings per share—diluted 37,672 37,811 37,868