Segment Information - Schedule of Goodwill and Intangible Assets Net of Accumulated Amortization (Detail) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Goodwill	$ 118.6	$ 147.3
Intangible Assets, Net of Accumulated Amortization	0.0	17.3
U.S. Operations
Segment Reporting Information [Line Items]
Goodwill	118.6	118.6
Intangible Assets, Net of Accumulated Amortization	0.0	0.0
International Operations
Segment Reporting Information [Line Items]
Goodwill	0.0	28.7
Intangible Assets, Net of Accumulated Amortization	0.0	$ 17.3
International Operations | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Argo Underwriting Agency Limited
Segment Reporting Information [Line Items]
Goodwill derecognized	$ 17.5