Business and Significant Accounting Policies - Intangible Assets and Accumulated Amortization (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 69.0	$ 112.2
Accumulated Amortization	51.7	51.7
Lloyd's capacity
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	17.3	60.5
Distribution network
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	45.5	45.5
Accumulated Amortization	45.5	45.5
Other
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	6.2	6.2
Accumulated Amortization	$ 6.2	$ 6.2