Information About Incurred and Paid Loss and LAE Development - Insurance Segment - RSUI - Property (Detail) $ in Millions		Dec. 31, 2021 USD ($) Claim		Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables		$ 12,331.5	[1]	$ 11,266.9	$ 10,344.5	$ 10,392.9
Insurance Segment
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	2,630.4
Insurance Segment | RSUI
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	2,063.9
Insurance Segment | RSUI | Property Insurance
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2012		4.3
Unpaid loss and LAE, net of reinsurance recoverables	[1]	415.5
Insurance Segment | RSUI | Property Insurance | Accident Year 2012
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		228.2		228.4	228.3	235.5	$ 236.1	$ 235.1	$ 256.1	$ 258.6	$ 262.5	$ 270.9
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 0.8
Cumulative Number of Reported Claims | Claim	[2]	2,313
Paid Loss and LAE, Net of Reinsurance		$ 222.5		219.4	219.2	216.7	216.2	202.4	193.5	181.9	157.5	$ 62.0
Insurance Segment | RSUI | Property Insurance | Accident Year 2013
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		153.5		154.4	154.3	152.5	152.4	152.1	150.4	157.2	157.3
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 1.1
Cumulative Number of Reported Claims | Claim	[2]	2,395
Paid Loss and LAE, Net of Reinsurance		$ 152.1		151.8	151.5	148.7	144.0	141.1	134.0	118.7	$ 72.7
Insurance Segment | RSUI | Property Insurance | Accident Year 2014
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		153.3		153.4	153.6	153.8	153.7	155.9	166.2	170.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 1.1
Cumulative Number of Reported Claims | Claim	[2]	3,094
Paid Loss and LAE, Net of Reinsurance		$ 151.9		151.6	151.3	150.3	148.8	145.0	133.8	$ 93.2
Insurance Segment | RSUI | Property Insurance | Accident Year 2015
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		140.4		140.2	140.1	141.3	142.6	136.1	140.5
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 1.5
Cumulative Number of Reported Claims | Claim	[2]	3,004
Paid Loss and LAE, Net of Reinsurance		$ 138.5		138.0	136.5	129.8	120.0	106.9	$ 70.8
Insurance Segment | RSUI | Property Insurance | Accident Year 2016
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		175.1		175.3	173.9	172.9	170.5	181.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 2.4
Cumulative Number of Reported Claims | Claim	[2]	3,368
Paid Loss and LAE, Net of Reinsurance		$ 169.8		167.3	161.2	147.1	127.1	$ 72.0
Insurance Segment | RSUI | Property Insurance | Accident Year 2017
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		296.7		300.0	304.1	306.5	330.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 10.6
Cumulative Number of Reported Claims | Claim	[2]	3,696
Paid Loss and LAE, Net of Reinsurance		$ 278.5		267.3	253.3	213.3	$ 88.6
Insurance Segment | RSUI | Property Insurance | Accident Year 2018
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		237.0		256.7	265.7	259.9
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 9.6
Cumulative Number of Reported Claims | Claim	[2]	3,071
Paid Loss and LAE, Net of Reinsurance		$ 216.1		202.9	175.1	$ 62.3
Insurance Segment | RSUI | Property Insurance | Accident Year 2019
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		205.1		200.9	204.3
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 13.9
Cumulative Number of Reported Claims | Claim	[2]	3,343
Paid Loss and LAE, Net of Reinsurance		$ 167.1		145.8	$ 79.8
Insurance Segment | RSUI | Property Insurance | Accident Year 2020
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		365.4		357.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 40.6
Cumulative Number of Reported Claims | Claim	[2]	4,257
Paid Loss and LAE, Net of Reinsurance		$ 251.2		$ 118.8
Insurance Segment | RSUI | Property Insurance | Accident Year 2021
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		341.5
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 80.6
Cumulative Number of Reported Claims | Claim	[2]	3,193
Paid Loss and LAE, Net of Reinsurance		$ 137.2
Insurance Segment | RSUI | Property Insurance | Accident Year 2012 Through 2021
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		2,296.2
Paid Loss and LAE, Net of Reinsurance		$ 1,885.0

[1] Includes unallocated LAE, which aggregate to 1.0 percent of gross loss and LAE reserves as of December 31, 2021. 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.