Information About Incurred and Paid Loss and LAE Development - Insurance Segment - RSUI - Casualty (Detail) $ in Millions		Dec. 31, 2019 USD ($) Claim		Dec. 31, 2018 USD ($)	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 ($)	Dec. 31, 2010 USD ($)
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables		$ 10,344.5	[1]	$ 10,392.9	$ 10,221.2	$ 9,851.0
Insurance Segment
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,928.2
Insurance Segment | RSUI
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,539.2
Insurance Segment | RSUI | Casualty Insurance
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2010		64.4
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,236.9
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2010
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		166.7		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		$ 7.5
Cumulative Number of Reported Claims | Claim	[2]	6,986
Paid Loss and LAE, Net of Reinsurance		$ 158.1		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		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		$ 13.3
Cumulative Number of Reported Claims | Claim	[2]	7,551
Paid Loss and LAE, Net of Reinsurance		$ 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		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		$ 31.9
Cumulative Number of Reported Claims | Claim	[2]	7,681
Paid Loss and LAE, Net of Reinsurance		$ 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		269.6		273.5	279.0	280.1	277.6	264.8	264.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 44.4
Cumulative Number of Reported Claims | Claim	[2]	8,731
Paid Loss and LAE, Net of Reinsurance		$ 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		310.9		320.3	322.4	321.1	322.7	292.0
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 56.8
Cumulative Number of Reported Claims | Claim	[2]	10,242
Paid Loss and LAE, Net of Reinsurance		$ 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		291.4		302.2	304.2	300.2	300.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 83.6
Cumulative Number of Reported Claims | Claim	[2]	9,375
Paid Loss and LAE, Net of Reinsurance		$ 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		312.0		301.3	293.7	290.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 83.0
Cumulative Number of Reported Claims | Claim	[2]	8,917
Paid Loss and LAE, Net of Reinsurance		$ 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		299.1		286.4	282.5
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 121.6
Cumulative Number of Reported Claims | Claim	[2]	8,716
Paid Loss and LAE, Net of Reinsurance		$ 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		301.5		290.0
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 185.6
Cumulative Number of Reported Claims | Claim	[2]	8,730
Paid Loss and LAE, Net of Reinsurance		$ 70.4		$ 13.0
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2019
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		316.9
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 267.7
Cumulative Number of Reported Claims | Claim	[2]	7,267
Paid Loss and LAE, Net of Reinsurance		$ 12.9
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2010 Through 2019
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		2,674.8
Paid Loss and LAE, Net of Reinsurance		$ 1,502.3

[1] Includes unallocated LAE, which aggregate to 1.1 percent of gross loss and LAE reserves as of December 31, 2019. 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.