Quarterly report pursuant to Section 13 or 15(d)

Capital (Details)

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Capital (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended
Mar. 16, 2018
Mar. 16, 2017
Mar. 31, 2018
Mar. 31, 2017
Class of Stock [Line Items]        
Payments of dividends $ 31.7 $ 10.7    
Dividend paid per sub-share (in usd per share)     $ 4.05 $ 1.35
Certificate        
Class of Stock [Line Items]        
Number of shares used in ratio (in shares)     1  
Sub Shares        
Class of Stock [Line Items]        
Number of shares used in ratio (in shares)     3,000  
Number of shares repurchased and retired (in shares)     13,146 29,496
Cash Dividend | Sub Shares        
Class of Stock [Line Items]        
Dividend paid per sub-share (in usd per share) $ 1.05 $ 0.35    
Special Dividend | Sub Shares        
Class of Stock [Line Items]        
Dividend paid per sub-share (in usd per share) $ 3 $ 1.00