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Operations and Summary of Significant Accounting Policies Recently Adopted Accounting Standards (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jan. 01, 2016
New Accounting Pronouncement, Early Adoption [Line Items]        
Cash paid on behalf of employees for shares withheld $ 5,341 $ 4,018 $ 3,716  
Deferred Income Tax and Other Long-term Liabilities [Member]        
New Accounting Pronouncement, Early Adoption [Line Items]        
Increase (decrease) deferred income taxes   12,100    
Deferred tax liabilities, net, noncurrent   16,500   $ 4,400
Other Noncurrent Assets [Member]        
New Accounting Pronouncement, Early Adoption [Line Items]        
Deferred tax assets, net, noncurrent   6,700   10,800
New Accounting Pronouncement, Early Adoption, Effect [Member]        
New Accounting Pronouncement, Early Adoption [Line Items]        
Deferred tax assets, net, current       $ (4,100)
Accounting Standards Update 2016-09 [Member]        
New Accounting Pronouncement, Early Adoption [Line Items]        
Excess tax benefit 1,100      
Cash paid on behalf of employees for shares withheld $ 1,300 $ 4,300