(Favorable) Unfavorable Prior Accident Year Loss Reserve Development (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2021		Dec. 31, 2020		Dec. 31, 2019
Claims Development [Line Items]
Claims incurred related to prior years		$ (249.7)		$ (220.8)		$ (184.7)
Reinsurance Segment
Claims Development [Line Items]
Claims incurred related to prior years		(249.6)		(206.9)		(195.8)
Reinsurance Segment | Property
Claims Development [Line Items]
Claims incurred related to prior years		(23.8)		(76.9)		(46.2)
Reinsurance Segment | Property | Catastrophe Events (Excluding Pandemic)
Claims Development [Line Items]
Claims incurred related to prior years		(35.5)	[1]	(43.3)	[2]	(6.8)	[3]
Reinsurance Segment | Property | Pandemic
Claims Development [Line Items]
Claims incurred related to prior years		62.6		0.0		0.0
Reinsurance Segment | Property | Non-catastrophe
Claims Development [Line Items]
Claims incurred related to prior years		(50.9)	[4]	(33.6)	[5]	(39.4)	[6]
Reinsurance Segment | Casualty & Specialty
Claims Development [Line Items]
Claims incurred related to prior years		(225.8)		(130.0)		(149.6)
Reinsurance Segment | Casualty & Specialty | Catastrophe Events (Excluding Pandemic)
Claims Development [Line Items]
Claims incurred related to prior years		(0.4)		(5.9)		(4.4)
Reinsurance Segment | Casualty & Specialty | Pandemic
Claims Development [Line Items]
Claims incurred related to prior years		(48.9)		0.0		0.0
Reinsurance Segment | Casualty & Specialty | Non-catastrophe
Claims Development [Line Items]
Claims incurred related to prior years		(176.5)	[7]	(124.1)	[8]	(145.2)	[9]
Insurance Segment | RSUI
Claims Development [Line Items]
Claims incurred related to prior years		(6.1)		(12.8)		(17.5)
Insurance Segment | RSUI | Casualty Insurance
Claims Development [Line Items]
Claims incurred related to prior years		5.0	[10]	1.8	[11]	(16.3)	[12]
Insurance Segment | RSUI | Property and Other Insurance
Claims Development [Line Items]
Claims incurred related to prior years		(11.1)	[13]	(14.6)	[14]	(1.2)	[15]
Insurance Segment | CapSpecialty Incorporated
Claims Development [Line Items]
Claims incurred related to prior years		6.0		(1.1)		28.6
Insurance Segment | CapSpecialty Incorporated | Ongoing Lines Of Business
Claims Development [Line Items]
Claims incurred related to prior years		6.1	[16]	(2.0)	[17]	26.1	[18]
Insurance Segment | CapSpecialty Incorporated | Terminated Program Business
Claims Development [Line Items]
Claims incurred related to prior years	[19]	(0.1)		0.9		3.0
Insurance Segment | CapSpecialty Incorporated | Asbestos and Environmental
Claims Development [Line Items]
Claims incurred related to prior years		$ 0.0		$ 0.0		$ (0.5)

[1] Primarily reflects favorable prior accident year loss reserve development related to catastrophic events in the 2018 accident year, partially offset by unfavorable prior accident year loss reserve development related to Hurricane Laura and Sally 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 wildfires in California in the 2017 and 2018 accident years, partially offset by unfavorable prior accident year loss reserve development related to Hurricane Irma in the 2017 accident year and Typhoon Faxai in the 2019 accident year.

[3] Primarily reflects favorable prior accident year loss reserve development related to wildfires in California in the 2018 accident year, partially offset by unfavorable prior accident year loss reserve development related to Typhoon Jebi in the 2018 accident year and Hurricane Irma in the 2017 accident year.

[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 2017 accident year.

[6] Primarily reflects favorable prior accident year loss reserve development in the 2016 and 2017 accident years, partially offset by unfavorable prior accident year loss reserve development in the 2018 accident year.

[7] Primarily reflects favorable prior accident year loss reserve development in the shorter-tailed lines of business in the 2020 accident year and in both the longer- and shorter-tailed lines of business in the 2015 and earlier accident years, partially offset by unfavorable prior accident year loss reserve development in the longer-tailed lines of business in the 2016 to 2018 accident years.

[8] Primarily reflects favorable prior accident year loss reserve development in the longer-tailed casualty lines of business in the 2014 and prior accident years and, to a lesser extent, shorter-tailed lines of business in the 2014 and prior accident years, partially offset by unfavorable prior accident year loss reserve development in the marine and aviation lines of business in the 2018 accident year.

[9] Primarily reflects favorable prior accident year loss reserve development in the longer-tailed casualty lines of business in the 2014 and prior accident years and, to a lesser extent, shorter-tailed lines of business in the 2014 and prior accident years, partially offset by unfavorable prior accident year loss reserve development in the marine and aviation lines of business in the 2018 accident year.

[10] Primarily reflects unfavorable prior accident year loss reserve development in the directors’ and officers’ liability lines of business in the 2012 and 2014 accident years and, to a lesser extent, the general liability and binding authority lines of business in earlier accident years, partially offset by favorable prior accident year loss reserve development in the umbrella/excess lines of business in the 2005 through 2015 accident years.

[11] Primarily reflects unfavorable prior accident year loss reserve development in the professional liability lines of business in the 2017 through 2019 accident years, partially offset by 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.

[12] 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 the 2016 through 2018 accident years.

[13] Primarily reflects favorable prior accident year loss reserve development related to losses not classified as catastrophes in recent accident years and, to a lesser extent, catastrophes in the 2017, 2018 and 2019 accident years, partially offset by unfavorable prior accident year loss reserve development related to catastrophes in the 2020 accident year.

[14] Primarily reflects favorable prior accident year loss reserve development related to U.S. catastrophe losses in the 2018 accident year and, to a lesser extent, favorable prior accident year loss reserve development related to the assumed property reinsurance lines of business from catastrophe losses in the 2019 and 2018 accident years.

[15] Primarily reflects favorable prior accident year loss reserve development related to Superstorm Sandy in the 2012 accident year, Hurricanes Florence and Michael in the 2018 accident year and Hurricanes Harvey and Maria in the 2017 accident year, partially offset by unfavorable prior accident year loss reserve development related to assumed property reinsurance lines of business from both catastrophe and non-catastrophe losses in the 2018 accident year.

[16] Primarily reflects unfavorable prior accident year loss reserve development in the healthcare, construction liability and other casualty lines of business in the 2015 through 2017 accident years.

[17] Primarily reflects favorable prior accident year loss reserve development in the surety lines of business in recent accident years.

[18] Primarily reflects unfavorable prior accident loss reserve development related to the professional liability and other casualty lines of business in the 2015 through 2018 accident years.

[19] Represents certain specialty lines of business written through a program administrator in connection with a terminated program in the 2010 and 2009 accident years and reflects favorable loss emergence compared with loss emergence patterns assumed in earlier periods for such business.