(Favorable) Unfavorable Prior Accident Year Loss Reserve Development (Detail) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2018		Jun. 30, 2017		Jun. 30, 2018		Jun. 30, 2017
Claims Development [Line Items]
Claims incurred related to prior years		$ (52.2)		$ (63.6)		$ (137.9)		$ (111.4)
Reinsurance Segment
Claims Development [Line Items]
Claims incurred related to prior years		(50.7)		(52.6)		(122.3)		(90.9)
Reinsurance Segment | Property
Claims Development [Line Items]
Claims incurred related to prior years		(13.7)		(22.9)		(55.1)		(54.4)
Reinsurance Segment | Property | Property Catastrophe
Claims Development [Line Items]
Claims incurred related to prior years		5.1	[1]	(4.0)	[2]	(25.3)	[3]	(4.4)	[2]
Reinsurance Segment | Property | Property Non-catastrophe
Claims Development [Line Items]
Claims incurred related to prior years		(18.8)	[4]	(18.9)	[5]	(29.8)	[4]	(50.0)	[5]
Reinsurance Segment | Casualty & Other
Claims Development [Line Items]
Claims incurred related to prior years		(37.0)		(29.7)		(67.2)		(36.5)
Reinsurance Segment | Casualty & Other | Ogden Rate Impact
Claims Development [Line Items]
Claims incurred related to prior years	[6]							24.4
Reinsurance Segment | Casualty & Other | Malpractice Treaties
Claims Development [Line Items]
Claims incurred related to prior years	[7]	(3.4)		(2.0)		(3.4)		(2.0)
Reinsurance Segment | Casualty & Other | All Other Casualty and Other
Claims Development [Line Items]
Claims incurred related to prior years		(33.6)	[8]	(27.7)	[9]	(63.8)	[10]	(58.9)	[9]
Insurance Segment | RSUI
Claims Development [Line Items]
Claims incurred related to prior years		(1.4)		(10.3)		(12.5)		(18.7)
Insurance Segment | RSUI | Casualty Insurance
Claims Development [Line Items]
Claims incurred related to prior years		(1.5)	[11]	(9.5)	[12]	(12.5)	[13]	(21.6)	[12]
Insurance Segment | RSUI | Property and Other Insurance
Claims Development [Line Items]
Claims incurred related to prior years		0.1		(0.8)				2.9	[14]
Insurance Segment | CapSpecialty Incorporated
Claims Development [Line Items]
Claims incurred related to prior years		$ (0.1)		(0.2)	[15]	$ (3.1)	[16]	(0.8)	[15]
Insurance Segment | Pacific Comp
Claims Development [Line Items]
Claims incurred related to prior years	[17]			$ (0.5)				$ (1.0)

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

[2] Primarily reflects favorable prior accident year loss reserve development related to catastrophes that occurred in the 2010 through 2016 accident years.

[3] Primarily reflects favorable prior accident year loss reserve development related to Hurricanes Harvey and Maria that occurred in the 2017 accident year and catastrophes that occurred in the 2016 accident year.

[4] Primarily reflects favorable prior accident year loss reserve development in the 2016 and 2017 accident years.

[5] Primarily reflects favorable prior accident year loss reserve development in the 2013 through 2016 accident years.

[6] Represents unfavorable prior accident year loss reserve development related to the U.K. Ministry of Justice's significant reduction in the discount rate, referred to as the Ogden rate, used to calculate lump-sum bodily injury payouts in personal injury insurance claims in the U.K to negative 0.75 percent as of March 20, 2017 from 2.50 percent.

[7] Represents certain medical 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 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.

[9] Primarily reflects favorable prior accident year loss reserve development in the longer-tailed U.S. professional liability lines of business in the 2005 through 2014 accident years, partially offset by unfavorable development in the shorter-tailed casualty lines of business in the 2015 accident year in the U.S. and in the U.K.

[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 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 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 2012 accident year.

[12] Primarily reflects favorable prior accident year loss reserve development in the umbrella/excess lines of business in the 2005 through 2011 accident years.

[13] 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 and 2012 accident years.

[14] Primarily reflects unfavorable prior accident year property loss reserve development in the binding authority lines of business in the 2015 and 2016 accident years.

[15] Primarily reflects favorable prior accident year loss reserve development in the casualty lines of business in the 2010 and 2015 accident years.

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

[17] Primarily reflects favorable prior accident year loss reserve development in the 2013 and prior accident years.