Liability for Loss and LAE - (Favorable) Unfavorable Prior Accident Year Loss Reserve Development (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022		Mar. 31, 2021
Claims Development [Line Items]
Claims incurred related to prior years	$ (53.3)		$ (56.1)
Reinsurance Segment
Claims Development [Line Items]
Claims incurred related to prior years	(49.9)		(53.5)
Reinsurance Segment | Property
Claims Development [Line Items]
Claims incurred related to prior years	(19.1)		13.8
Reinsurance Segment | Property | Catastrophe Events (Excluding Pandemic)
Claims Development [Line Items]
Claims incurred related to prior years	(2.4)	[1]	(4.5)	[2]
Reinsurance Segment | Property | Pandemic
Claims Development [Line Items]
Claims incurred related to prior years	(4.3)		29.8
Reinsurance Segment | Property | Non-catastrophe
Claims Development [Line Items]
Claims incurred related to prior years	(12.4)	[3]	(11.5)	[4]
Reinsurance Segment | Casualty & Specialty
Claims Development [Line Items]
Claims incurred related to prior years	(30.8)		(67.3)
Reinsurance Segment | Casualty & Specialty | Catastrophe Events (Excluding Pandemic)
Claims Development [Line Items]
Claims incurred related to prior years	(1.7)		(1.3)
Reinsurance Segment | Casualty & Specialty | Pandemic
Claims Development [Line Items]
Claims incurred related to prior years	0.9		(11.8)
Reinsurance Segment | Casualty & Specialty | Non-catastrophe
Claims Development [Line Items]
Claims incurred related to prior years	(30.0)	[5]	(54.2)	[6]
Insurance Segment | RSUI
Claims Development [Line Items]
Claims incurred related to prior years	(6.3)		(2.7)
Insurance Segment | RSUI | Casualty Insurance
Claims Development [Line Items]
Claims incurred related to prior years	(1.8)	[7]	(1.1)	[8]
Insurance Segment | RSUI | Property and Other Insurance
Claims Development [Line Items]
Claims incurred related to prior years	(4.5)	[9]	(1.6)	[10]
Insurance Segment | CapSpecialty Incorporated
Claims Development [Line Items]
Claims incurred related to prior years	$ 2.9	[11]	$ 0.1	[12]

[1] Primarily reflects favorable prior accident year loss reserve development related to several catastrophic events in the 2018 and 2019 accident years.

[2] Primarily reflects favorable prior accident year loss reserve development related to Hurricane Michael in the 2018 accident year, partially offset by unfavorable prior accident year loss reserve development related to Hurricane Laura in the 2020 accident year.

[3] Primarily reflects favorable prior accident year loss reserve development in the 2018 through 2020 accident years.

[4] Primarily reflects favorable prior accident year loss reserve development in the 2020 accident year.

[5] Primarily reflects favorable prior accident year loss reserve development in the longer-tailed lines of business in the 2008 and prior accident years and shorter-tailed lines of business in the 2019 through 2021 accident years, partially offset by unfavorable prior accident year development in the longer-tailed lines of business in the 2009 through 2018 accident years.

[6] Primarily reflects favorable prior accident year loss reserve development in the longer-tailed lines of business in the 2016 and earlier accident years and
shorter-tailed lines of business in the 2019 and 2020 accident years, partially offset by unfavorable development in certain longer- and shorter-tailed
lines of business in the 2018 accident year.

[7] Primarily reflects favorable prior accident year loss reserve development in in the directors' and officers' liability lines of business in the 2018 and earlier accident years and the umbrella/excess lines of business in the 2015 and earlier accident years.

[8] Primarily reflects favorable prior accident year loss reserve development in the umbrella/excess lines of business in the 2007 through 2014 accident years, partially offset by unfavorable prior accident year loss reserve development in the directors' and officers' liability lines of business in the 2013 accident year.

[9] Primarily reflects favorable prior accident year loss reserve development related to catastrophe losses in the 2021 accident year and, to a lesser extent, the 2020 accident year.

[10] Primarily reflects favorable prior accident year loss reserve development related to catastrophes in the 2017 and 2018 accident years, partially offset by unfavorable prior accident year loss reserve development related to catastrophes in the 2020 accident year.

[11] Primarily reflects unfavorable prior accident year loss reserve development in several casualty liability lines of business in the 2014 through 2018 accident years.

[12] Primarily reflects unfavorable prior accident year loss reserve development in several casualty lines of business.