(Favorable) Unfavorable Prior Accident Year Loss Reserve Development (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020		Mar. 31, 2019
Claims Development [Line Items]
Claims incurred related to prior years		$ (59.0)		$ (37.4)
Reinsurance Segment
Claims Development [Line Items]
Claims incurred related to prior years		(57.7)		(36.4)
Reinsurance Segment | Property
Claims Development [Line Items]
Claims incurred related to prior years		(46.7)		(10.7)
Reinsurance Segment | Property | Property Catastrophe
Claims Development [Line Items]
Claims incurred related to prior years		(32.3)	[1]	8.4	[2]
Reinsurance Segment | Property | Property Non-catastrophe
Claims Development [Line Items]
Claims incurred related to prior years		(14.4)	[3]	(19.1)	[4]
Reinsurance Segment | Casualty & Other
Claims Development [Line Items]
Claims incurred related to prior years		(11.0)		(25.7)
Reinsurance Segment | Casualty & Other | All Other Casualty and Other
Claims Development [Line Items]
Claims incurred related to prior years		(11.0)	[5]	(25.7)	[6]
Insurance Segment | RSUI
Claims Development [Line Items]
Claims incurred related to prior years		(1.5)		(3.7)
Insurance Segment | RSUI | Casualty Insurance
Claims Development [Line Items]
Claims incurred related to prior years		(1.0)	[7]	(5.6)	[8]
Insurance Segment | RSUI | Property and Other Insurance
Claims Development [Line Items]
Claims incurred related to prior years		(0.5)	[9]	1.9	[10]
Insurance Segment | CapSpecialty Incorporated
Claims Development [Line Items]
Claims incurred related to prior years	[11]	$ 0.2		$ 2.7

[1] 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.

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

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

[4] Primarily reflects favorable prior accident year loss reserve development in the 2016 through 2018 accident years.

[5] 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.

[6] Primarily reflects favorable prior accident year loss reserve development in the shorter-tailed lines of business in the 2012 through 2017 prior accident years and in the professional liability lines of business in the 2007 and prior accident years.

[7] 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.

[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 the 2017 accident year.

[9] 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.

[10] Primarily reflects unfavorable prior accident year loss reserve development related to Hurricanes Harvey in the 2017 accident year and Florence in the 2018 accident year.

[11] 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.