(Favorable) Unfavorable Prior Accident Year Loss Reserve Development (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021		Mar. 31, 2020
Claims Development [Line Items]
Claims incurred related to prior years	$ (56.1)		$ (59.0)
Reinsurance Segment
Claims Development [Line Items]
Claims incurred related to prior years	(53.5)		(57.7)
Reinsurance Segment | Property
Claims Development [Line Items]
Claims incurred related to prior years	13.8		(46.7)
Reinsurance Segment | Property | Catastrophe Events (Excluding Pandemic)
Claims Development [Line Items]
Claims incurred related to prior years	(4.5)	[1]	(32.3)	[2]
Reinsurance Segment | Property | Pandemic
Claims Development [Line Items]
Claims incurred related to prior years	29.8		0.0
Reinsurance Segment | Property | Non-catastrophe
Claims Development [Line Items]
Claims incurred related to prior years	(11.5)	[3]	(14.4)	[4]
Reinsurance Segment | Casualty & Other
Claims Development [Line Items]
Claims incurred related to prior years	(67.3)		(11.0)
Reinsurance Segment | Casualty & Other | Catastrophe Events (Excluding Pandemic)
Claims Development [Line Items]
Claims incurred related to prior years	(1.3)		1.4
Reinsurance Segment | Casualty & Other | Pandemic
Claims Development [Line Items]
Claims incurred related to prior years	(11.8)		0.0
Reinsurance Segment | Casualty & Other | Non-catastrophe
Claims Development [Line Items]
Claims incurred related to prior years	(54.2)	[5]	(12.4)	[6]
Insurance Segment | RSUI
Claims Development [Line Items]
Claims incurred related to prior years	(2.7)		(1.5)
Insurance Segment | RSUI | Casualty Insurance
Claims Development [Line Items]
Claims incurred related to prior years	(1.1)	[7]	(1.0)	[8]
Insurance Segment | RSUI | Property and Other Insurance
Claims Development [Line Items]
Claims incurred related to prior years	(1.6)	[9]	(0.5)	[10]
Insurance Segment | CapSpecialty Incorporated
Claims Development [Line Items]
Claims incurred related to prior years	$ 0.1	[11]	$ 0.2	[12]

[1] 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

[2] Primarily reflects favorable prior accident year loss reserve development related to Typhoon Hagibis in the 2019 accident year and catastrophes in the 2018 accident year.

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

[4] Primarily reflects favorable prior accident year loss reserve development in the 2017 and 2019 accident years.

[5] Primarily reflects favorable prior accident year loss reserve development in the longer-tailed lines of business in the 2016 and prior 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.

[6] Primarily reflects favorable prior accident year loss reserve development in the longer-tailed lines of business in the 2013 and earlier accident years, partially offset by unfavorable prior accident year development in the shorter-tailed lines of business in the 2014 through 2019 accident years.

[7] 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.

[8] Primarily reflects favorable prior accident year loss reserve development in the directors’ and officers’ liability and umbrella/excess lines of business in the 2011 through 2015 accident years, partially offset by unfavorable prior accident year loss reserve development in the professional liability lines of business in recent accident years.

[9] 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.

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

[11] Primarily reflects unfavorable prior accident year loss reserve development related to several casualty lines of business.

[12] Primarily reflects unfavorable prior accident year loss reserve development in certain specialty lines of business written through a program administrator in connection with a terminated program in the 2009 and 2010 accident years.