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Shareholders' Equity (Narrative) (Details) - Class A Common Stock [Member] - USD ($)
$ in Millions
Mar. 31, 2021
Nov. 02, 2020
New Plan [Member]    
Class of Stock [Line Items]    
Stock repurchase program, authorized   $ 1,000.0
Stock repurchase program, remaining authorization $ 475.1  
Prior Plan [Member]    
Class of Stock [Line Items]    
Stock repurchase program, remaining authorization   $ 124.3