(Favorable) Unfavorable Prior Accident Year Loss Reserve Development (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018		Mar. 31, 2017
Claims Development [Line Items]
Claims incurred related to prior years		$ (85.7)		$ (47.7)
Reinsurance Segment
Claims Development [Line Items]
Claims incurred related to prior years		(71.6)		(38.3)
Reinsurance Segment | Property
Claims Development [Line Items]
Claims incurred related to prior years		(41.4)		(31.5)
Reinsurance Segment | Property | Property Catastrophe
Claims Development [Line Items]
Claims incurred related to prior years		(30.4)	[1]	(0.4)	[2]
Reinsurance Segment | Property | Property Non-catastrophe
Claims Development [Line Items]
Claims incurred related to prior years		(11.0)	[3]	(31.1)	[4]
Reinsurance Segment | Casualty & Other
Claims Development [Line Items]
Claims incurred related to prior years		(30.2)		(6.8)
Reinsurance Segment | Casualty & Other | Ogden Rate Impact
Claims Development [Line Items]
Claims incurred related to prior years	[5]			24.4
Reinsurance Segment | Casualty & Other | All Other Casualty and Other
Claims Development [Line Items]
Claims incurred related to prior years		(30.2)	[6]	(31.2)	[7]
Insurance Segment | RSUI
Claims Development [Line Items]
Claims incurred related to prior years		(11.1)		(8.4)
Insurance Segment | RSUI | Casualty Insurance
Claims Development [Line Items]
Claims incurred related to prior years		(11.0)	[8]	(12.2)	[9]
Insurance Segment | RSUI | Property and Other Insurance
Claims Development [Line Items]
Claims incurred related to prior years		(0.1)		3.8	[10]
Insurance Segment | CapSpecialty Incorporated
Claims Development [Line Items]
Claims incurred related to prior years		$ (3.0)	[11]	(0.5)	[12]
Insurance Segment | Pacific Comp
Claims Development [Line Items]
Claims incurred related to prior years	[13]			$ (0.5)

[1] Reflects favorable prior accident year loss reserve development primarily related to Hurricanes Maria and Harvey that occurred in the 2017 accident year.

[2] Reflects favorable prior accident year loss reserve development from several catastrophes that occurred in the 2010 through 2016 accident years.

[3] Reflects favorable prior accident year loss reserve development primarily related to the 2016 and 2017 accident years.

[4] Reflects favorable prior accident year loss reserve development primarily related to the 2014 through 2016 accident years

[5] Represents unfavorable prior accident year loss reserve development arising from the U.K. Ministry of Justice's decision to significantly reduce 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. As of March 20, 2017, the Ogden rate changed from 2.50 percent to negative 0.75 percent.

[6] Primarily reflects favorable prior accident year loss reserve development in the shorter-tailed lines of business related to the 2016 and 2017 accident years, and in the shorter-tailed and longer-tailed lines of business related to the 2005 through 2013 accident years.

[7] Generally reflects favorable prior accident year loss reserve development in a variety of casualty & other lines of business primarily related to the 2010 through 2016 accident years.

[8] Primarily reflects favorable prior accident year loss reserve development in the umbrella/excess lines of business related to 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 related primarily to the 2009 accident year.

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

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

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

[12] Primarily reflects favorable prior accident year loss reserve development in the casualty lines of business related to the 2005, 2006 and 2012 accident years.

[13] Primarily reflects favorable prior accident year loss reserve development related to the 2013 accident year.