(Favorable) Unfavorable Prior Accident Year Loss Reserve Development (Detail) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2019		Jun. 30, 2018		Jun. 30, 2019		Jun. 30, 2018
Claims Development [Line Items]
Claims incurred related to prior years		$ (56.6)		$ (52.2)		$ (94.0)		$ (137.9)
Reinsurance Segment
Claims Development [Line Items]
Claims incurred related to prior years		(44.8)		(50.7)		(81.2)		(122.3)
Reinsurance Segment | Property
Claims Development [Line Items]
Claims incurred related to prior years		(16.2)		(13.7)		(26.9)		(55.1)
Reinsurance Segment | Property | Property Catastrophe
Claims Development [Line Items]
Claims incurred related to prior years		(9.9)	[1]	5.1	[2]	(1.5)	[3]	(25.3)	[4]
Reinsurance Segment | Property | Property Non-catastrophe
Claims Development [Line Items]
Claims incurred related to prior years		(6.3)	[5]	(18.8)	[6]	(25.4)	[7]	(29.8)	[6]
Reinsurance Segment | Casualty & Other
Claims Development [Line Items]
Claims incurred related to prior years		(28.6)		(37.0)		(54.3)		(67.2)
Reinsurance Segment | Casualty & Other | Malpractice Treaties
Claims Development [Line Items]
Claims incurred related to prior years	[8]	(1.4)		(3.4)		(1.4)		(3.4)
Reinsurance Segment | Casualty & Other | All Other Casualty and Other
Claims Development [Line Items]
Claims incurred related to prior years		(27.2)	[9]	(33.6)	[10]	(52.9)	[11]	(63.8)	[12]
Insurance Segment | RSUI
Claims Development [Line Items]
Claims incurred related to prior years		(12.5)		(1.4)		(16.2)		(12.5)
Insurance Segment | RSUI | Casualty Insurance
Claims Development [Line Items]
Claims incurred related to prior years		(12.1)	[13]	(1.5)	[14]	(17.7)	[13]	(12.5)	[14]
Insurance Segment | RSUI | Property and Other Insurance
Claims Development [Line Items]
Claims incurred related to prior years		(0.4)		0.1		1.5	[15]
Insurance Segment | CapSpecialty Incorporated
Claims Development [Line Items]
Claims incurred related to prior years		$ 0.7	[16]	$ (0.1)		$ 3.4	[17]	$ (3.1)	[18]

[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 Hurricane Maria in the 2017 accident year, partially offset by favorable prior accident year loss reserve development related to Hurricane Harvey in the 2017 accident year and catastrophes in the 2016 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 Hurricanes Harvey and Maria in the 2017 accident year and catastrophes in the 2016 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 2016 and 2017 accident years.

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

[8] 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.

[9] Primarily reflects favorable prior accident year loss reserve development in the longer-tailed lines of business in the 2013 and prior accident years.

[10] Primarily reflects favorable prior accident year loss reserve development in the shorter-tailed casualty lines of business in the 2015 and 2017 accident years and the longer-tailed lines of business in the 2008 through 2010 accident years.

[11] Primarily reflects favorable prior accident year loss reserve development in the longer-tailed lines of business in the 2013 and prior accident years and the shorter-tailed lines of business in the 2012 through 2014 accident years.

[12] 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 the longer-tailed lines of business in the 2008 through 2010 accident years.

[13] 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.

[14] 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 recent accident years.

[15] 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 Hurricane Harvey in the 2017 accident year.

[16] Primarily reflects unfavorable prior accident year loss reserve development in the professional liability lines of business in the 2016 accident year.

[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.

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