Information About Incurred and Paid Loss and LAE Development - Insurance Segment - RSUI - Property (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 | Property Insurance
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2009		2.6
Unpaid loss and LAE, net of reinsurance recoverables	[1]	359.9
Insurance Segment | RSUI | Property Insurance | Accident Year 2009
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		64.9		65.0	64.7	64.5	$ 63.7	$ 63.1	$ 62.5	$ 60.1	$ 63.1	$ 78.3
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 0.7
Cumulative Number of Reported Claims | Claim	[2]	1,315
Paid Loss and LAE, Net of Reinsurance		$ 64.0		63.9	63.6	63.3	61.9	61.4	58.9	54.3	51.0	$ 36.6
Insurance Segment | RSUI | Property Insurance | Accident Year 2010
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		101.7		109.6	109.9	109.8	104.2	105.7	101.8	101.7	110.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 0.8
Cumulative Number of Reported Claims | Claim	[2]	1,630
Paid Loss and LAE, Net of Reinsurance		$ 100.8		101.3	101.5	101.1	100.6	98.6	92.4	83.6	$ 53.0
Insurance Segment | RSUI | Property Insurance | Accident Year 2011
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		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		$ 1.0
Cumulative Number of Reported Claims | Claim	[2]	2,205
Paid Loss and LAE, Net of Reinsurance		$ 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		235.5		236.1	235.1	256.1	258.6	262.5	270.9
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 8.2
Cumulative Number of Reported Claims | Claim	[2]	2,310
Paid Loss and LAE, Net of Reinsurance		$ 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		152.5		152.4	152.1	150.4	157.2	157.3
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 2.1
Cumulative Number of Reported Claims | Claim	[2]	2,393
Paid Loss and LAE, Net of Reinsurance		$ 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		153.8		153.7	155.9	166.2	170.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 2.7
Cumulative Number of Reported Claims | Claim	[2]	3,090
Paid Loss and LAE, Net of Reinsurance		$ 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		141.3		142.6	136.1	140.5
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 4.6
Cumulative Number of Reported Claims | Claim	[2]	3,001
Paid Loss and LAE, Net of Reinsurance		$ 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		172.9		170.5	181.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 7.7
Cumulative Number of Reported Claims | Claim	[2]	3,346
Paid Loss and LAE, Net of Reinsurance		$ 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		306.5		330.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 35.5
Cumulative Number of Reported Claims | Claim	[2]	2,627
Paid Loss and LAE, Net of Reinsurance		$ 213.3		$ 88.6
Insurance Segment | RSUI | Property Insurance | Accident Year 2018
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		259.9
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 106.8
Cumulative Number of Reported Claims | Claim	[2]	2,686
Paid Loss and LAE, Net of Reinsurance		$ 62.3
Insurance Segment | RSUI | Property Insurance | Accident Year 2009 Through 2018
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		1,748.9
Paid Loss and LAE, Net of Reinsurance		$ 1,391.6

[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.