Business and Significant Accounting Policies - Intangible Assets and Accumulated Amortization (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 147.3	$ 147.3
Accumulated Amortization	53.8	50.5
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	47.0	44.1
Additional Lloyd's Capacity [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	4.8	4.8
Accumulated Amortization	4.8	4.8
Other [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	3.3	3.3
Accumulated Amortization	$ 2.0	$ 1.6