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Share-Based Compensation and Pensions - Other Share-Based Compensation Plans (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Northshore incentive plan | Time-based restricted shares      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Unrecognized compensation cost $ 0.9    
Unrecognized compensation cost, period of recognition 4 months 9 days    
Northshore incentive plan | Minimum | Time-based restricted shares      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Vesting period 2 years    
Northshore incentive plan | Maximum | Time-based restricted shares      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Vesting period 3 years    
StarStone incentive plan | Time-based restricted shares      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Unrecognized compensation cost $ 1.8    
Unrecognized compensation cost, period of recognition 2 years    
StarStone incentive plan | Minimum | Time-based restricted shares      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Vesting period 2 years    
StarStone incentive plan | Maximum | Time-based restricted shares      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Vesting period 3 years    
Deferred compensation and ordinary share plan for non-employee directors | Director | Share-based compensation plans      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Restricted share units credited to the accounts of non-employee directors (shares) 5,976 5,691 3,852
Employee share purchase plan | Employee share purchase plan      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Discount rate from market price 15.00%    
Maximum subscription rate 15.00%    
Stock issued to employees (shares) 15,269 14,183 12,401