(Favorable) Unfavorable Prior Accident Year Development on Loss Reserves (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	10 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Insurance Reserves [Line Items]
Claims incurred related to prior years		$ (215.2)		$ (203.0)		$ (12.3)	[1]
Reinsurance Segment
Insurance Reserves [Line Items]
Favorable prior accident year development on loss and LAE reserves acquired in the merger, included in current year net loss	(87.8)					(87.8)
Claims incurred related to prior years		(182.4)		(224.1)
RSUI | Insurance Segment
Insurance Reserves [Line Items]
Claims incurred related to prior years		(35.4)		(17.9)		(31.1)	[1]
Property | Reinsurance Segment
Insurance Reserves [Line Items]
Claims incurred related to prior years		(73.1)		(184.7)
Property | Reinsurance Segment | Superstorm Sandy
Insurance Reserves [Line Items]
Claims incurred related to prior years		(1.6)		(73.7)
Property | Reinsurance Segment | Other Catastrophe
Insurance Reserves [Line Items]
Claims incurred related to prior years		(15.7)		(35.6)
Property and Other Insurance | RSUI | Insurance Segment
Insurance Reserves [Line Items]
Claims incurred related to prior years		(5.3)	[2]	8.0	[3]	17.0	[1],[4]
Property and Other Insurance | RSUI | Insurance Segment | Hurricane Ike
Insurance Reserves [Line Items]
Claims incurred related to prior years						9.7
Property and Other Insurance | RSUI | Insurance Segment | Hurricane Katrina
Insurance Reserves [Line Items]
Claims incurred related to prior years				$ 8.5

[1] $87.8 million of favorable prior accident year development on loss and LAE reserves acquired in the merger are included in current year net loss and LAE for the reinsurance segment.

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

[3] Includes $8.5 million of unfavorable development from Hurricane Katrina, which resulted from a significant claim that settled for more than previously estimated due to an adverse court ruling, partially offset by net favorable development with respect to other property reserves.

[4] Includes $9.7 million of unfavorable development from Hurricane Ike in 2008.