(Favorable) Unfavorable Prior Accident Year Loss Reserve Development (Detail) - USD ($) $ in Millions		3 Months Ended				9 Months Ended
		Sep. 30, 2019		Sep. 30, 2018		Sep. 30, 2019		Sep. 30, 2018
Claims Development [Line Items]
Claims incurred related to prior years		$ (58.9)		$ (75.0)		$ (152.9)		$ (212.8)
Reinsurance Segment
Claims Development [Line Items]
Claims incurred related to prior years		(59.6)		(41.5)		(140.8)		(163.7)
Reinsurance Segment | Property
Claims Development [Line Items]
Claims incurred related to prior years		(23.3)		(2.8)		(50.2)		(57.8)
Reinsurance Segment | Property | Property Catastrophe
Claims Development [Line Items]
Claims incurred related to prior years		(12.5)	[1]	9.6	[2]	(14.0)	[3]	(15.6)	[4]
Reinsurance Segment | Property | Property Non-catastrophe
Claims Development [Line Items]
Claims incurred related to prior years		(10.8)	[5]	(12.4)	[6]	(36.2)	[5]	(42.2)	[6]
Reinsurance Segment | Casualty & Other
Claims Development [Line Items]
Claims incurred related to prior years		(36.3)		(38.7)		(90.6)		(105.9)
Reinsurance Segment | Casualty & Other | Malpractice Treaties
Claims Development [Line Items]
Claims incurred related to prior years	[7]					(1.4)		(3.4)
Reinsurance Segment | Casualty & Other | All Other Casualty and Other
Claims Development [Line Items]
Claims incurred related to prior years		(36.3)	[8]	(38.7)	[9]	(89.2)	[8]	(102.5)	[10]
Insurance Segment | RSUI
Claims Development [Line Items]
Claims incurred related to prior years		(1.2)		(32.0)		(17.4)		(44.5)
Insurance Segment | RSUI | Casualty Insurance
Claims Development [Line Items]
Claims incurred related to prior years		(0.2)	[11]	(4.3)	[12]	(17.9)	[11]	(16.8)	[12]
Insurance Segment | RSUI | Property and Other Insurance
Claims Development [Line Items]
Claims incurred related to prior years		(1.0)		(27.7)	[13]	0.5	[14]	(27.7)	[13]
Insurance Segment | CapSpecialty Incorporated
Claims Development [Line Items]
Claims incurred related to prior years		$ 1.9	[15]	$ (1.5)	[16]	$ 5.3	[17]	$ (4.6)	[16]

[1] Primarily reflects favorable prior accident year loss reserve development related to wildfires in California in the 2018 accident year.

[2] Primarily reflects unfavorable prior accident year loss reserve development related to Hurricanes Maria and Irma in the 2017 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.

[4] Primarily reflects favorable prior accident year loss reserve development related to Hurricane Harvey in the 2017 accident year and catastrophes in the 2016 accident year, partially offset by unfavorable prior accident year loss reserve development related to Hurricanes Maria and Irma in the 2017 accident year.

[5] Primarily reflects favorable prior accident year loss reserve development in the 2018 accident year.

[6] Primarily reflects favorable prior accident year loss reserve development in the 2017 accident year.

[7] Represents certain malpractice treaties pursuant to which the increased underwriting profits created by the favorable prior accident year loss reserve development are largely retained by the cedants. As a result, the favorable prior accident year loss reserve development is largely offset by an increase in profit commission expense incurred when such favorable prior accident year loss reserve development occurs.

[8] Primarily reflects favorable prior accident year loss reserve development in the longer-tailed lines of business in the 2015 and prior accident years, partially offset by unfavorable prior accident year development in the 2016 through 2018 accident years, largely from shorter-tailed lines of business.

[9] Primarily reflects favorable prior accident year loss reserve development in the longer-tailed casualty lines of business in the 2007 and prior accident years, partially offset by unfavorable prior accident year loss reserve development in the longer-tailed casualty lines of business in the 2014 through 2016 accident years.

[10] Primarily reflects favorable prior accident year loss reserve development in the shorter-tailed casualty lines of business in the 2016 and 2017 accident years and in the longer-tailed casualty lines of business in the 2010 and prior accident years, partially offset by unfavorable prior accident year loss reserve development in the longer-tailed casualty lines of business in the 2014 accident year.

[11] 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.

[12] Primarily reflects favorable prior accident year loss reserve development in the umbrella/excess lines of business in the 2005 through 2012 accident years, partially offset by unfavorable prior accident year loss reserve development in the directors’ and officers’ liability lines of business in the 2009, 2012 and 2016 accident years.

[13] Primarily reflects favorable prior accident year loss reserve development related to Hurricane Irma in the 2017 accident year and, to a lesser extent, Hurricane Matthew in the 2016 accident year, as well as various other losses not classified as catastrophes in recent accident years.

[14] Primarily reflects unfavorable prior accident year loss reserve development related to the assumed property reinsurance lines of business from catastrophe losses in recent accident years, partially offset by favorable prior accident year loss reserve development related to Superstorm Sandy in the 2012 accident year and by Hurricanes Harvey and Maria in the 2017 accident year.

[15] Primarily reflects unfavorable prior accident year loss reserve development in the professional liability lines of business in recent accident years.

[16] Primarily reflects favorable prior loss reserve development related to the surety lines of business in the 2016 and 2017 accident years.

[17] 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 and, to a lesser extent, unfavorable prior accident year loss reserve development in the professional liability lines of business in recent accident years.