Information About Incurred and Paid Loss and LAE Development - Reinsurance Segment - Property (Detail) - USD ($) $ in Millions		Dec. 31, 2021		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
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
Reinsurance Segment
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	9,701.1
Reinsurance Segment | Property
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2012		19.7
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,958.3
Reinsurance Segment | Property | Accident Year 2012
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		456.1		454.9	460.3	462.5	$ 469.4	$ 478.1	$ 495.6	$ 530.3	$ 579.1	$ 697.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		0.0
Paid Loss and LAE, Net of Reinsurance		466.6		461.1	460.1	451.2	448.0	438.0	416.7	377.8	268.9	$ 90.3
Reinsurance Segment | Property | Accident Year 2013
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		394.0		394.7	398.1	403.6	406.8	422.1	444.6	470.6	501.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		0.0
Paid Loss and LAE, Net of Reinsurance		403.9		403.0	401.4	399.7	396.7	389.8	361.0	277.4	$ 113.1
Reinsurance Segment | Property | Accident Year 2014
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		416.2		419.7	425.1	429.8	435.2	451.3	464.8	496.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		0.0
Paid Loss and LAE, Net of Reinsurance		399.4		398.4	397.3	394.0	382.9	360.0	297.6	$ 109.4
Reinsurance Segment | Property | Accident Year 2015
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		295.5		297.9	301.9	306.3	320.6	332.0	368.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		1.4
Paid Loss and LAE, Net of Reinsurance		311.9		308.8	307.6	303.1	278.3	217.7	$ 96.0
Reinsurance Segment | Property | Accident Year 2016
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		578.1		577.8	587.9	596.7	647.2	684.1
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		2.6
Paid Loss and LAE, Net of Reinsurance		563.9		552.2	535.1	504.8	429.8	$ 210.5
Reinsurance Segment | Property | Accident Year 2017
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		1,114.8		1,127.7	1,148.3	1,179.5	1,174.6
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		20.2
Paid Loss and LAE, Net of Reinsurance		1,039.9		1,023.0	977.9	818.5	$ 324.5
Reinsurance Segment | Property | Accident Year 2018
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		1,116.8		1,147.8	1,169.4	1,152.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		49.1
Paid Loss and LAE, Net of Reinsurance		1,002.3		944.2	811.8	$ 309.5
Reinsurance Segment | Property | Accident Year 2019
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		972.2		981.5	988.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		56.0
Paid Loss and LAE, Net of Reinsurance		793.2		645.5	$ 265.1
Reinsurance Segment | Property | Accident Year 2020
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		1,253.5		1,207.5
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		297.1
Paid Loss and LAE, Net of Reinsurance		732.1		$ 301.1
Reinsurance Segment | Property | Accident Year 2021
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		1,468.1
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		640.3
Paid Loss and LAE, Net of Reinsurance		413.5
Reinsurance Segment | Property | Accident Year 2012 Through 2021
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		8,065.3
Paid Loss and LAE, Net of Reinsurance		$ 6,126.7

[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.