Capital (Details) $ / shares in Units, $ in Millions  | 
3 Months Ended | 9 Months Ended | ||||
|---|---|---|---|---|---|---|
| 
 Mar. 16, 2020  
USD ($)  
$ / shares 
 | 
 Mar. 15, 2019  
USD ($)  
$ / shares 
 | 
 Sep. 30, 2020  
$ / shares 
 | 
 Sep. 30, 2019  
$ / shares 
 | 
 Sep. 30, 2020  
$ / shares  
shares 
 | 
 Sep. 30, 2019  
$ / shares  
shares 
 | 
|
| Class of Stock [Line Items] | ||||||
| Payments of dividends | $ | $ 124.1 | $ 46.5 | ||||
| Dividend paid per sub-share (in dollars per share) | $ 0 | $ 0 | $ 16.00 | $ 6.00 | ||
| Sub Shares | ||||||
| Class of Stock [Line Items] | ||||||
| Ratio of certificates to sub-shares | 0.0003333 | |||||
| Number of shares repurchased and retired (in shares) | shares | 0 | 6,258 | ||||
| Cash Dividend | Sub Shares | ||||||
| Class of Stock [Line Items] | ||||||
| Dividend paid per sub-share (in dollars per share) | $ 10.00 | $ 1.75 | ||||
| Special Dividend | Sub Shares | ||||||
| Class of Stock [Line Items] | ||||||
| Dividend paid per sub-share (in dollars per share) | $ 6.00 | $ 4.25 | ||||
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- Definition Number Of Shares Used In Ratio1 No definition available. 
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. No definition available. 
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- Definition Aggregate dividends paid during the period for each share of common stock outstanding. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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- Definition Amount of cash outflow in the form of ordinary dividends to common shareholders of the parent entity. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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- Definition Number of shares that have been repurchased and retired during the period. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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