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Changes in Stockholders' Equity (Narrative) (Details)
3 Months Ended
Mar. 31, 2018
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Purchases and retirement of common stock $ 7,100,000
Stock repurchase program, outstanding balance authorized to be repurchased 324,300,000
Stock repurchase program, value of remaining number of shares authorized to be repurchased $ 31,400,000
Open Market Transaction [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Stock Repurchased and Retired During Period, Shares | shares 110,132
Purchases and retirement of common stock $ 7,100,000
Stock Repurchased and Retired During Period, Average Cost Per Share | $ / shares $ 64.35
December 2016 ASR Program [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Stock repurchase program, authorized amount $ 292,900,000.0