Information About Incurred and Paid Loss and LAE Development - Insurance Segment - RSUI - Property (Detail) $ in Millions		Dec. 31, 2020 USD ($) Claim		Dec. 31, 2019 USD ($)	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 ($)	Dec. 31, 2011 USD ($)
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables		$ 11,266.9	[1]	$ 10,344.5	$ 10,392.9	$ 10,221.2
Insurance Segment
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	2,283.6
Insurance Segment | RSUI
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,814.7
Insurance Segment | RSUI | Property Insurance
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2011		3.7
Unpaid loss and LAE, net of reinsurance recoverables	[1]	408.5
Insurance Segment | RSUI | Property Insurance | Accident Year 2011
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		160.2		160.2	159.9	159.3	$ 159.3	$ 159.0	$ 159.9	$ 160.5	$ 162.0	$ 168.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 0.9
Cumulative Number of Reported Claims | Claim	[2]	2,207
Paid Loss and LAE, Net of Reinsurance		$ 159.0		158.8	158.6	157.6	157.2	156.1	154.3	144.0	118.4	$ 61.0
Insurance Segment | RSUI | Property Insurance | Accident Year 2012
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		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		$ 1.1
Cumulative Number of Reported Claims | Claim	[2]	2,312
Paid Loss and LAE, Net of Reinsurance		$ 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		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		$ 1.5
Cumulative Number of Reported Claims | Claim	[2]	2,395
Paid Loss and LAE, Net of Reinsurance		$ 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		153.4		153.6	153.8	153.7	155.9	166.2	170.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 1.4
Cumulative Number of Reported Claims | Claim	[2]	3,091
Paid Loss and LAE, Net of Reinsurance		$ 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		140.2		140.1	141.3	142.6	136.1	140.5
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 2.3
Cumulative Number of Reported Claims | Claim	[2]	3,004
Paid Loss and LAE, Net of Reinsurance		$ 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		175.3		173.9	172.9	170.5	181.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 3.1
Cumulative Number of Reported Claims | Claim	[2]	3,367
Paid Loss and LAE, Net of Reinsurance		$ 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		300.0		304.1	306.5	330.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 17.6
Cumulative Number of Reported Claims | Claim	[2]	3,689
Paid Loss and LAE, Net of Reinsurance		$ 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		256.7		265.7	259.9
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 28.3
Cumulative Number of Reported Claims | Claim	[2]	3,033
Paid Loss and LAE, Net of Reinsurance		$ 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		200.9		204.3
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 25.2
Cumulative Number of Reported Claims | Claim	[2]	3,266
Paid Loss and LAE, Net of Reinsurance		$ 145.8		$ 79.8
Insurance Segment | RSUI | Property Insurance | Accident Year 2020
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		357.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 77.7
Cumulative Number of Reported Claims | Claim	[2]	3,750
Paid Loss and LAE, Net of Reinsurance		$ 118.8
Insurance Segment | RSUI | Property Insurance | Accident Year 2011 Through 2020
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		2,126.7
Paid Loss and LAE, Net of Reinsurance		$ 1,721.9

[1] Includes unallocated LAE, which aggregate to 1.0 percent of gross loss and LAE reserves as of December 31, 2020. 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.