(Favorable) Unfavorable Prior Accident Year Loss Reserve Development (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017		Mar. 31, 2016
Claims Development [Line Items]
Claims incurred related to prior years		$ (47.7)		$ (85.3)
Reinsurance Segment
Claims Development [Line Items]
Claims incurred related to prior years		(38.3)		(71.0)
Reinsurance Segment | Property
Claims Development [Line Items]
Claims incurred related to prior years		(31.5)		(33.2)
Reinsurance Segment | Property | Property Catastrophe
Claims Development [Line Items]
Claims incurred related to prior years		(0.4)	[1]	(1.0)	[2]
Reinsurance Segment | Property | Property Non-catastrophe
Claims Development [Line Items]
Claims incurred related to prior years		(31.1)	[3]	(32.2)	[4]
Reinsurance Segment | Casualty & Other
Claims Development [Line Items]
Claims incurred related to prior years		(6.8)		(37.8)
Reinsurance Segment | Casualty & Other | Ogden Rate Impact
Claims Development [Line Items]
Claims incurred related to prior years	[5]	24.4
Reinsurance Segment | Casualty & Other | The Medical Malpractice Treaties
Claims Development [Line Items]
Claims incurred related to prior years	[6]			(6.9)
Reinsurance Segment | Casualty & Other | All Other Casualty and Other
Claims Development [Line Items]
Claims incurred related to prior years		(31.2)	[7]	(30.9)	[8]
Insurance Segment | RSUI
Claims Development [Line Items]
Claims incurred related to prior years		(8.4)		(12.8)
Insurance Segment | RSUI | Casualty Insurance
Claims Development [Line Items]
Claims incurred related to prior years		(12.2)	[9]	(9.8)	[10]
Insurance Segment | RSUI | Property and Other Insurance
Claims Development [Line Items]
Claims incurred related to prior years		3.8	[11]	(3.0)	[12]
Insurance Segment | CapSpecialty Incorporated
Claims Development [Line Items]
Claims incurred related to prior years		(0.5)		(1.5)
Insurance Segment | CapSpecialty Incorporated | Asbestos and Environmental
Claims Development [Line Items]
Claims incurred related to prior years				(2.0)
Insurance Segment | CapSpecialty Incorporated | All Other CapSpecialty Incorporated
Claims Development [Line Items]
Claims incurred related to prior years		(0.5)	[13]	$ 0.5
Insurance Segment | Pacific Comp
Claims Development [Line Items]
Claims incurred related to prior years	[14]	$ (0.5)

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

[2] Reflects favorable prior accident year loss reserve development from several catastrophes that occurred in the 2010 through 2015 accident years.

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

[4] Reflects favorable prior accident year loss reserve development primarily related to the 2014 and 2015 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 (0.75) percent.

[6] 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.

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

[8] Generally reflects favorable prior accident year loss reserve development in a variety of casualty & other lines of business primarily from the 2009, 2014 and 2015 accident years, partially offset by unfavorable prior accident year loss reserve development from the 2013 accident year.

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

[10] Primarily reflects favorable prior accident year loss reserve development in the general liability, umbrella/excess and professional liability lines of business related to the 2006 through 2012 accident years.

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

[12] Primarily reflects favorable prior accident year loss reserve development in non-catastrophe property lines of business in recent accident years.

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

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