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Employee benefit plans and compensation plans - Share-Based and Other Long-Term Incentive Compensation Plans (Details)
$ in Millions
12 Months Ended
Nov. 01, 2018
USD ($)
Aug. 22, 2018
shares
Dec. 31, 2018
USD ($)
item
shares
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based compensation expense | $     $ 2.5    
Non-employee independent directors          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based compensation expense | $     $ 0.4    
Vesting period     1 year    
Shares granted     24,475    
2016 Long Term Incentive Plan          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based compensation expense | $     $ 1.0 $ 3.9 $ 16.6
Vesting period     3 years    
Amount of incentive awards paid to employees received from White Mountains | $     $ 1.4 $ 2.5 $ 5.4
Total number of shares reserved for issuance     5,614,904    
2018 Omnibus plan          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period     3 years    
Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based compensation expense | $     $ 1.5    
Performance Shares | 2016 Long Term Incentive Plan          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of outstanding cycles | item     3    
IPO Incentive Awards | 2018 Omnibus plan          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based compensation expense | $     $ 0.6    
Number of shares purchased     203,589    
Total number of shares reserved for issuance     552,594    
Number of years over which shares to be purchased     3 years    
Lock in period of shares (in years)     3 years    
Number of shares awarded for shares purchased through various share-based compensation plans     1    
Total pool of shares granted at target level of performance on or before November 1, 2018 | $ $ 13.0        
IPO Incentive Awards | 2018 Omnibus plan | 4.5% ROE          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Payout as percentage of awarded shares     0.00%    
IPO Incentive Awards | 2018 Omnibus plan | 9.0% ROE          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Payout as percentage of awarded shares     100.00%    
IPO Incentive Awards | 2018 Omnibus plan | 13.5% ROE          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Payout as percentage of awarded shares     150.00%    
IPO Incentive Awards | 2018 Omnibus plan | 2019 ROE          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting (Percentage)     25.00%    
Shares to be purchased and held out of the pledged shares by the participants (as a percent)     33.00%    
IPO Incentive Awards | 2018 Omnibus plan | 2020 ROE          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting (Percentage)     25.00%    
Shares to be purchased and held out of the pledged shares by the participants (as a percent)     67.00%    
IPO Incentive Awards | 2018 Omnibus plan | 2021 ROE          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting (Percentage)     25.00%    
Shares to be purchased and held out of the pledged shares by the participants (as a percent)     100.00%    
IPO Incentive Awards | 2018 Omnibus plan | 2019-2021 Average ROE          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting (Percentage)     25.00%    
ESPP          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Maximum number of shares to be purchased   1,000      
Minimum number of shares to be purchased   100      
Number of shares purchased     149,236    
Net promissory notes | $     $ 0.9    
ESPP | For the first 100 shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Purchase price of common shares as a percentage on its market value   85.00%      
ESPP | For the next 900 shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Purchase price of common shares as a percentage on its market value   100.00%      
ESPP | Maximum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of shares available for purchase under the plan   1,000,000