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Income Taxes - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Income Tax [Line Items]          
Effective income tax expense (benefit) rate 17.50% 5.00% 26.90% 11.20%  
Alternative minimum tax credit carry forward         $ 11.0
Net operating loss carryforwards $ 24.9   $ 24.9   21.9
Section 163(j) carryforward 6.8   6.8   $ 9.0
Alternative Minimum Tax Credits          
Income Tax [Line Items]          
Income tax refunds received $ 5.5   $ 11.0    
UNITED STATES          
Income Tax [Line Items]          
Statutory income tax rates 21.00%   21.00%    
BERMUDA          
Income Tax [Line Items]          
Statutory income tax rates 0.00%   0.00%    
CAYMAN ISLANDS          
Income Tax [Line Items]          
Statutory income tax rates     0.00%    
UNITED KINGDOM          
Income Tax [Line Items]          
Statutory income tax rates     19.00%    
IRELAND | Non Trading Income          
Income Tax [Line Items]          
Statutory income tax rates     25.00%    
IRELAND | Capital Gain          
Income Tax [Line Items]          
Statutory income tax rates     33.00%    
IRELAND | Trading Income          
Income Tax [Line Items]          
Statutory income tax rates     12.50%