Information About Incurred and Paid Loss and LAE Development - Reinsurance Segment - Property (Detail) - USD ($) $ in Millions		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	Dec. 31, 2011
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
Reinsurance Segment
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	8,983.3
Reinsurance Segment | Property
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2011		24.4
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,606.0
Reinsurance Segment | Property | Accident Year 2011
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		1,206.4		1,207.7	1,209.6	1,210.9	$ 1,217.1	$ 1,235.2	$ 1,240.7	$ 1,269.4	$ 1,342.3	$ 1,351.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		0.0
Paid Loss and LAE, Net of Reinsurance		1,197.0		1,195.3	1,199.7	1,195.0	1,187.2	1,171.0	1,129.2	1,014.3	796.4	$ 407.8
Reinsurance Segment | Property | Accident Year 2012
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		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		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		394.7		398.1	403.6	406.8	422.1	444.6	470.6	501.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		0.7
Paid Loss and LAE, Net of Reinsurance		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		419.7		425.1	429.8	435.2	451.3	464.8	496.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		2.9
Paid Loss and LAE, Net of Reinsurance		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		297.9		301.9	306.3	320.6	332.0	368.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		4.2
Paid Loss and LAE, Net of Reinsurance		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		577.8		587.9	596.7	647.2	684.1
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		11.1
Paid Loss and LAE, Net of Reinsurance		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		1,127.7		1,148.3	1,179.5	1,174.6
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		31.9
Paid Loss and LAE, Net of Reinsurance		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		1,147.8		1,169.4	1,152.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		85.2
Paid Loss and LAE, Net of Reinsurance		944.2		811.8	$ 309.5
Reinsurance Segment | Property | Accident Year 2019
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		981.5		988.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		132.9
Paid Loss and LAE, Net of Reinsurance		645.5		$ 265.1
Reinsurance Segment | Property | Accident Year 2020
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		1,207.5
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		648.7
Paid Loss and LAE, Net of Reinsurance		301.1
Reinsurance Segment | Property | Accident Year 2011 Through 2020
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		7,815.9
Paid Loss and LAE, Net of Reinsurance		$ 6,234.3

[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.