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7. Stockholders' Equity Treasury Shares (Details) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 12 Months Ended
Nov. 30, 2019
Aug. 31, 2019
Jun. 30, 2019
May 31, 2019
Nov. 30, 2018
Dec. 31, 2019
Dec. 31, 2018
Equity, Class of Treasury Stock [Line Items]              
Treasury Stock, Shares 281,636 259,293 244,548 218,571 193,150    
Stock Repurchase Program, Remaining Authorized Repurchase Amount $ 173.1 $ 181.1 $ 186.1 $ 193.7 $ 201.1    
Stock Repurchase Program, Authorized Amount           $ 250.0  
Treasury Stock, Shares, Acquired 22,343 14,745 25,977 25,421 178,911    
Treasury Stock Acquired, Average Cost Per Share $ 358.04 $ 339.05 $ 292.01 $ 291.70 $ 273.39    
Treasury Class-A [Member]              
Equity, Class of Treasury Stock [Line Items]              
Stock Issued During Period, Shares, Treasury Stock Reissued           (118,910)  
Treasury Class B [Member]              
Equity, Class of Treasury Stock [Line Items]              
Treasury Stock, Shares           0 917
Employee Stock [Member]              
Equity, Class of Treasury Stock [Line Items]              
Stock Issued During Period, Shares, Treasury Stock Reissued           (19,755)