Information About Incurred and Paid Loss and LAE Development - Insurance Segment - RSUI - Casualty (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 | Casualty Insurance
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2011		61.9
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,406.2
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2011
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		183.2		184.9	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		$ 10.9
Cumulative Number of Reported Claims | Claim	[2]	7,565
Paid Loss and LAE, Net of Reinsurance		$ 157.4		156.4	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		210.2		221.8	231.9	233.8	238.9	242.8	230.3	226.3	226.3
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 17.0
Cumulative Number of Reported Claims | Claim	[2]	7,708
Paid Loss and LAE, Net of Reinsurance		$ 178.5		176.1	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		261.4		269.6	273.5	279.0	280.1	277.6	264.8	264.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 33.6
Cumulative Number of Reported Claims | Claim	[2]	8,759
Paid Loss and LAE, Net of Reinsurance		$ 212.9		203.9	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		300.3		310.9	320.3	322.4	321.1	322.7	292.0
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 37.3
Cumulative Number of Reported Claims | Claim	[2]	10,260
Paid Loss and LAE, Net of Reinsurance		$ 249.1		234.6	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		289.8		291.4	302.2	304.2	300.2	300.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 67.3
Cumulative Number of Reported Claims | Claim	[2]	9,455
Paid Loss and LAE, Net of Reinsurance		$ 196.2		176.5	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		317.5		312.0	301.3	293.7	290.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 68.7
Cumulative Number of Reported Claims | Claim	[2]	9,034
Paid Loss and LAE, Net of Reinsurance		$ 220.8		197.8	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		311.1		299.1	286.4	282.5
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 101.7
Cumulative Number of Reported Claims | Claim	[2]	8,924
Paid Loss and LAE, Net of Reinsurance		$ 163.1		115.6	63.6	$ 9.4
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2018
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		310.0		301.5	290.0
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 130.9
Cumulative Number of Reported Claims | Claim	[2]	9,181
Paid Loss and LAE, Net of Reinsurance		$ 116.8		70.4	$ 13.0
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2019
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		332.2		316.9
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 225.8
Cumulative Number of Reported Claims | Claim	[2]	8,367
Paid Loss and LAE, Net of Reinsurance		$ 57.9		$ 12.9
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2020
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		393.9
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 343.3
Cumulative Number of Reported Claims | Claim	[2]	6,671
Paid Loss and LAE, Net of Reinsurance		$ 12.6
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2011 Through 2020
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		2,909.6
Paid Loss and LAE, Net of Reinsurance		$ 1,565.3

[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.