Information About Incurred and Paid Loss and LAE Development - Insurance Segment - RSUI - Casualty (Detail) $ in Millions		Dec. 31, 2018 USD ($) Claim		Dec. 31, 2017 USD ($)	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 ($)	Dec. 31, 2009 USD ($)
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables		$ 10,392.9	[1]	$ 10,221.2	$ 9,851.0	$ 9,629.9
Insurance Segment
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,914.4
Insurance Segment | RSUI
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,606.8
Insurance Segment | RSUI | Casualty Insurance
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2009		65.6
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,246.9
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2009
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		201.0		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		$ 16.8
Cumulative Number of Reported Claims | Claim	[2]	6,884
Paid Loss and LAE, Net of Reinsurance		$ 183.2		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		168.0		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		$ 10.5
Cumulative Number of Reported Claims | Claim	[2]	6,908
Paid Loss and LAE, Net of Reinsurance		$ 152.3		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		189.7		202.0	206.8	211.6	212.1	208.3	205.9	205.9
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 27.1
Cumulative Number of Reported Claims | Claim	[2]	7,516
Paid Loss and LAE, Net of Reinsurance		$ 153.6		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		231.9		233.8	238.9	242.8	230.3	226.3	226.3
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 46.9
Cumulative Number of Reported Claims | Claim	[2]	7,623
Paid Loss and LAE, Net of Reinsurance		$ 166.7		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		273.5		279.0	280.1	277.6	264.8	264.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 62.4
Cumulative Number of Reported Claims | Claim	[2]	8,677
Paid Loss and LAE, Net of Reinsurance		$ 191.3		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		320.3		322.4	321.1	322.7	292.0
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 82.8
Cumulative Number of Reported Claims | Claim	[2]	10,173
Paid Loss and LAE, Net of Reinsurance		$ 206.5		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		302.2		304.2	300.2	300.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 125.5
Cumulative Number of Reported Claims | Claim	[2]	9,255
Paid Loss and LAE, Net of Reinsurance		$ 129.2		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		301.3		293.7	290.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 125.6
Cumulative Number of Reported Claims | Claim	[2]	8,698
Paid Loss and LAE, Net of Reinsurance		$ 123.6		69.2	$ 13.7
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2017
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		286.4		282.5
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 175.9
Cumulative Number of Reported Claims | Claim	[2]	8,273
Paid Loss and LAE, Net of Reinsurance		$ 63.6		$ 9.4
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2018
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		290.0
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 246.6
Cumulative Number of Reported Claims | Claim	[2]	7,529
Paid Loss and LAE, Net of Reinsurance		$ 13.0
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2009 Through 2018
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		2,564.3
Paid Loss and LAE, Net of Reinsurance		$ 1,383.0

[1] Includes unallocated LAE, which aggregate to 1.1 percent of gross loss and LAE reserves as of December 31, 2018. 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.