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 9 Months Ended
Mar. 16, 2018
Mar. 16, 2017
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Class of Stock [Line Items]            
Payments of dividends $ 31.7 $ 10.7        
Dividend paid per sub-share (in dollars per share)     $ 0 $ 0 $ 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)         39,768 88,272
Cash Dividend | Sub Shares            
Class of Stock [Line Items]            
Dividend paid per sub-share (in dollars per share) $ 1.05 $ 0.35        
Special Dividend | Sub Shares            
Class of Stock [Line Items]            
Dividend paid per sub-share (in dollars per share) $ 3 $ 1.00