(Favorable) Unfavorable Prior Accident Year Loss Reserve Development (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Claims Development [Line Items]
Claims incurred related to prior years		$ (368.0)		$ (215.5)		$ (215.2)
Reinsurance Segment
Claims Development [Line Items]
Claims incurred related to prior years		(293.5)		(208.3)		(182.4)
Reinsurance Segment | Property
Claims Development [Line Items]
Claims incurred related to prior years		(105.7)		(76.7)		(73.1)
Reinsurance Segment | Casualty & Other
Claims Development [Line Items]
Claims incurred related to prior years		(187.8)		(131.6)		(109.3)
Insurance Segment | RSUI
Claims Development [Line Items]
Claims incurred related to prior years		(68.3)		(11.9)		(35.4)
Property Catastrophe | Reinsurance Segment | Property
Claims Development [Line Items]
Claims incurred related to prior years		(14.2)	[1]	(28.0)	[2]	(17.3)	[3]
Property Catastrophe | Super Storm Sandy | Reinsurance Segment | Property
Claims Development [Line Items]
Claims incurred related to prior years				(27.7)		(1.6)
Property Catastrophe | Other Catastrophe | Reinsurance Segment | Property
Claims Development [Line Items]
Claims incurred related to prior years						(15.7)
All Other Casualty and Other | Reinsurance Segment | Casualty & Other
Claims Development [Line Items]
Claims incurred related to prior years		(177.0)	[4]	(157.7)	[5]	(96.6)	[6]
All Other Casualty and Other | Reinsurance Segment | Casualty & Other | French Medical Malpractice Commutation with European Client
Claims Development [Line Items]
Claims incurred related to prior years	$ (30.7)
Property and Other Insurance | Insurance Segment | RSUI
Claims Development [Line Items]
Claims incurred related to prior years		(33.0)	[7]	(9.0)	[8]	$ (5.3)	[9]
Property and Other Insurance | Super Storm Sandy | Insurance Segment | RSUI
Claims Development [Line Items]
Claims incurred related to prior years		$ (20.6)		$ (4.1)

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

[2] Includes favorable prior accident year loss reserve development of ($27.7) million from Super Storm Sandy in 2012 and various smaller amounts on catastrophes that occurred in the 2010, 2011, 2013 and 2014 accident years, partially offset by unfavorable prior accident year development from the New Zealand earthquake in 2010.

[3] Includes favorable prior accident year loss reserve development of ($1.6) million from Super Storm Sandy in 2012 and ($15.7) million of net favorable prior accident year loss reserve development from other catastrophes. The ($15.7) million primarily reflects favorable prior accident year loss reserve development from several catastrophes that occurred primarily in the 2011 and 2013 accident years, partially offset by unfavorable prior accident year loss reserve development from the New Zealand earthquake in 2010.

[4] Generally reflects favorable prior accident year loss reserve development in a variety of casualty & other lines of business primarily from the 2003 through 2015 accident years.

[5] Generally reflects favorable prior accident year loss reserve development in a variety of casualty & other lines of business primarily from the 2005 through 2014 accident years, including ($30.7) million of favorable prior accident year development related to French medical malpractice loss reserves commuted in the fourth quarter of 2015 with a European cedant, partially offset by unfavorable prior accident year development from the 2004 and prior accident years.

[6] Generally reflects favorable prior accident year loss reserve development in a variety of casualty & other lines of business primarily from the 2003 through 2007 and 2010 through 2011 accident years, partially offset by unfavorable prior accident year loss reserve development from the 2013 accident year and the 2002 and prior accident years.

[7] Includes favorable prior accident year loss reserve development of ($20.6) million from Super Storm Sandy in 2012 and various other smaller amounts primarily from the non-catastrophe property lines of business in recent accident years.

[8] Primarily reflects favorable prior accident year development of ($4.1) million from Super Storm Sandy in 2012, net of reinsurance, and favorable prior accident year loss reserve development related to unallocated LAE reserves.

[9] Primarily reflects favorable prior accident year development on unallocated LAE reserves and prior year catastrophe loss reserves from recent accident years.