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Common shareholder's equity, mezzanine equity, and non controlling interests - Mezzanine equity (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Nov. 05, 2018
Mezzanine equity      
Redeemable preference shares with accrued dividend carrying value $ 223.0 $ 232.2  
Series B preference shares      
Mezzanine equity      
Preferred shares authorized (in shares) 15,000,000   15,000,000
Preferred shares par value (in dollars per share) $ 0.01   $ 0.01
Preferential rights, Minimum gross proceeds from the issuance of senior or pari passu shares $ 100.0    
Redeemable preference shares with accrued dividend carrying value $ 223.0 $ 232.2  
Series B preference shares | Private Placement | Preference Share Investors      
Mezzanine equity      
Share subscriptions (in shares)     11,901,670
Series A redeemable preference shares      
Mezzanine equity      
Redeemable preference shares outstanding (in shares)     100,000
Redemption amount of preference shares     $ 95.0
Gain on redemption of preference shares     $ 13.8
Series A redeemable preference shares | IMGAH      
Mezzanine equity      
Preferred shares authorized (in shares) 150,000    
Number of convertible preferred shares issued as a consideration (in shares) 100,000    
Redeemable preference share liquidation preference per share (in dollars per share) $ 1,000