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Shareholders' Equity (Narrative) (Details) - Class A Common Stock [Member] - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2017
May 04, 2017
Jan. 12, 2017
Jun. 04, 2015
Previous Stock Repurchase Program [Member]        
Class of Stock [Line Items]        
Stock repurchase program, authorized       $ 1,000.0
Stock repurchased, shares 42,163      
Stock repurchased, value $ 4.4      
Weighted average price per share $ 104.81      
Stock repurchase program, remaining authorization     $ 150.0  
New Stock Repurchase Program [Member]        
Class of Stock [Line Items]        
Stock repurchase program, authorized     $ 1,000.0  
New Stock Repurchase Program [Member] | Subsequent Event [Member]        
Class of Stock [Line Items]        
Stock repurchase program, remaining authorization   $ 1,000.0