(Favorable) Unfavorable Prior Accident Year Loss Reserve Development (Parenthetical) (Detail) - USD ($) $ in Millions			12 Months Ended
	Mar. 20, 2017	Mar. 19, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Claims Development [Line Items]
Claims incurred related to prior years			$ (329.0)		$ (298.6)		$ (368.0)
Ogden discount rate	(0.75%)	2.50%
Insurance Segment | RSUI
Claims Development [Line Items]
Claims incurred related to prior years			(58.7)		(43.2)		(68.3)
Property and Other Insurance | Insurance Segment | RSUI
Claims Development [Line Items]
Claims incurred related to prior years			$ (30.9)	[1]	$ (4.3)	[2]	(33.0)	[3]
Property and Other Insurance | Super Storm Sandy | Insurance Segment | RSUI
Claims Development [Line Items]
Claims incurred related to prior years							$ (20.6)

[1] Primarily reflects favorable prior accident year loss reserve development related to Hurricane Irma in the 2017 accident year and, to a lesser extent, Hurricane Matthew in the 2016 accident year, as well as various other losses not classified as catastrophes in recent accident years.

[2] Primarily reflects favorable catastrophe prior accident year loss reserve development in the 2016 accident year, partially offset by unfavorable prior accident year property loss reserve development in the binding authority lines of business primarily in the 2015 and 2016 accident years.

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