Information About Incurred and Paid Loss and LAE Development - Insurance Segment - RSUI - Casualty (Detail) $ in Millions		Dec. 31, 2017 USD ($) Claim		Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables		$ 10,221.2	[1]	$ 9,851.0	$ 9,629.9	$ 10,307.8
Insurance Segment
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,837.9
Insurance Segment | RSUI
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,567.4
Insurance Segment | RSUI | Casualty Insurance
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2008		60.9
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,211.8
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2008
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		208.6		216.9	220.9	230.8	$ 237.0	$ 245.3	$ 254.8	$ 260.6	$ 255.0	$ 255.0
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 17.7
Cumulative Number of Reported Claims | Claim	[2]	7,144
Paid Loss and LAE, Net of Reinsurance		$ 182.2		179.4	171.3	164.2	157.4	145.3	124.4	89.5	35.1	$ 9.5
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2009
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		199.4		212.0	221.8	232.2	236.3	236.3	234.4	233.7	230.1
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 21.8
Cumulative Number of Reported Claims | Claim	[2]	6,879
Paid Loss and LAE, Net of Reinsurance		$ 174.1		166.4	157.5	149.5	136.9	101.9	73.2	38.0	$ 7.2
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2010
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		171.0		177.0	178.6	194.6	203.4	204.1	204.1	204.1
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 19.7
Cumulative Number of Reported Claims | Claim	[2]	6,891
Paid Loss and LAE, Net of Reinsurance		$ 149.2		132.9	128.6	122.6	90.1	70.1	30.9	$ 4.7
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2011
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		202.0		206.8	211.6	212.1	208.3	205.9	205.9
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 41.0
Cumulative Number of Reported Claims | Claim	[2]	7,493
Paid Loss and LAE, Net of Reinsurance		$ 150.0		138.5	118.4	100.3	66.7	31.9	$ 6.5
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2012
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		233.8		238.9	242.8	230.3	226.3	226.3
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 60.5
Cumulative Number of Reported Claims | Claim	[2]	7,597
Paid Loss and LAE, Net of Reinsurance		$ 159.3		144.0	125.5	96.0	38.4	$ 6.8
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2013
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		279.0		280.1	277.6	264.8	264.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 75.5
Cumulative Number of Reported Claims | Claim	[2]	8,609
Paid Loss and LAE, Net of Reinsurance		$ 176.6		146.2	103.4	50.0	$ 10.1
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2014
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		322.4		321.1	322.7	292.0
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 101.1
Cumulative Number of Reported Claims | Claim	[2]	10,101
Paid Loss and LAE, Net of Reinsurance		$ 179.1		130.1	69.5	$ 13.0
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2015
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		304.2		300.2	300.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 159.4
Cumulative Number of Reported Claims | Claim	[2]	9,050
Paid Loss and LAE, Net of Reinsurance		$ 96.6		47.3	$ 9.0
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2016
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		293.7		290.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 178.7
Cumulative Number of Reported Claims | Claim	[2]	8,280
Paid Loss and LAE, Net of Reinsurance		$ 69.2		$ 13.7
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2017
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		282.5
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 245.9
Cumulative Number of Reported Claims | Claim	[2]	7,003
Paid Loss and LAE, Net of Reinsurance		$ 9.4
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2008 Through 2017
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		2,496.6
Paid Loss and LAE, Net of Reinsurance		$ 1,345.7

[1] Includes unallocated LAE, which aggregate to 1.3 percent of gross loss and LAE reserves as of December 31, 2017. Net loss and LAE Reserves by component are shown in the preceding tables, and consolidated gross loss and LAE reserves is presented in the Consolidated Balance Sheets.

[2] Represents claims reported by insured claimants.