(Favorable) Unfavorable Prior Accident Year Loss Reserve Development (Detail) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2021		Jun. 30, 2020		Jun. 30, 2021		Jun. 30, 2020
Claims Development [Line Items]
Claims incurred related to prior years	$ (85.5)		$ (34.7)		$ (141.5)		$ (93.7)
Reinsurance Segment
Claims Development [Line Items]
Claims incurred related to prior years	(76.4)		(51.2)		(129.8)		(108.9)
Reinsurance Segment | Property
Claims Development [Line Items]
Claims incurred related to prior years	(3.8)		(13.0)		10.0		(59.7)
Reinsurance Segment | Property | Catastrophe Events (Excluding Pandemic)
Claims Development [Line Items]
Claims incurred related to prior years	1.4	[1]	0.5		(3.0)	[2]	(31.9)	[3]
Reinsurance Segment | Property | Pandemic
Claims Development [Line Items]
Claims incurred related to prior years	17.8		0.0		47.6		0.0
Reinsurance Segment | Property | Non-catastrophe
Claims Development [Line Items]
Claims incurred related to prior years	(23.0)	[4]	(13.5)	[5]	(34.6)	[4]	(27.8)	[5]
Reinsurance Segment | Casualty & Specialty
Claims Development [Line Items]
Claims incurred related to prior years	(72.6)		(38.2)		(139.8)		(49.2)
Reinsurance Segment | Casualty & Specialty | Catastrophe Events (Excluding Pandemic)
Claims Development [Line Items]
Claims incurred related to prior years	0.1		(3.8)		(1.2)		(2.4)
Reinsurance Segment | Casualty & Specialty | Pandemic
Claims Development [Line Items]
Claims incurred related to prior years	(18.5)		0.0		(30.3)		0.0
Reinsurance Segment | Casualty & Specialty | Non-catastrophe
Claims Development [Line Items]
Claims incurred related to prior years	(54.2)	[6]	(34.4)	[7]	(108.3)	[8]	(46.8)	[9]
Insurance Segment | RSUI
Claims Development [Line Items]
Claims incurred related to prior years	(9.3)		16.5		(12.0)		15.0
Insurance Segment | RSUI | Casualty Insurance
Claims Development [Line Items]
Claims incurred related to prior years	(2.6)	[10]	16.5	[11]	(3.7)	[10]	15.5	[11]
Insurance Segment | RSUI | Property and Other Insurance
Claims Development [Line Items]
Claims incurred related to prior years	(6.7)	[12]	0.0		(8.3)	[12]	(0.5)
Insurance Segment | CapSpecialty Incorporated
Claims Development [Line Items]
Claims incurred related to prior years	$ 0.2	[13]	$ 0.0		$ 0.3	[13]	$ 0.2

[1] Primarily reflects unfavorable prior accident year loss reserve development related to Hurricanes Laura and Sally in the 2020 accident year, partially offset by favorable prior accident year loss reserve development related to catastrophic events in earlier accident years.

[2] Primarily reflects favorable prior accident year loss reserve development related to catastrophic events in the 2010, 2017 and 2018 accident years, partially offset by unfavorable prior accident year loss reserve development related to Hurricanes Laura and Sally in the 2020 accident year.

[3] Primarily reflects favorable prior accident year loss reserve development related to Typhoon Hagibis in the 2019 accident year and, to a lesser extent, catastrophic events in the 2018 accident year.

[4] Primarily reflects favorable prior accident year loss reserve development related to the 2019 and 2020 accident years.

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

[6] Primarily reflects favorable prior accident year loss reserve development in the longer-tailed lines of business in the 2015 and earlier accident years, and the shorter-tailed lines of business in the 2016, 2018 and 2020 accident years.

[7] Primarily reflects favorable prior accident year loss reserve development in the longer-tailed lines of business in the 2016 and earlier accident years and in the shorter-tailed lines of business in the 2017 accident year, partially offset by unfavorable prior accident year development in the shorter-tailed lines of business in the 2018 and 2019 accident years.

[8] Primarily reflects favorable prior accident year loss reserve development in the longer-tailed lines of business in the 2015 and earlier accident years, and the shorter-tailed lines of business in the 2019 and 2020 accident years.

[9] Primarily reflects favorable prior accident year loss reserve development in the longer-tailed lines of business in the 2013 and earlier accident years and in the shorter-tailed lines of business in the 2017 accident year, partially offset by unfavorable prior accident year development in the shorter-tailed lines of business in the 2014 through 2016, 2018 and 2019 accident years.

[10] 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 and 2014 accident years and the binding authority lines of business in the 2011 through 2013 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 related to losses not classified as catastrophes in recent accident years and, to a lesser extent, 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.

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