Business and Significant Accounting Policies - Intangible Assets and Accumulated Amortization (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 147.3	$ 147.3
Accumulated Amortization	55.5	53.8
Lloyd's Capacity [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	89.0	89.0
Distribution Network [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	50.2	50.2
Accumulated Amortization	48.2	47.0
Other [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	8.1	8.1
Accumulated Amortization	$ 7.3	$ 6.8