Information About Incurred and Paid Loss and LAE Development - Reinsurance Segment - Casualty & Other - Shorter-Tailed Lines of Business (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 | Casualty & Other | Shorter Tailed Lines Of Business
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2011	[2]	141.7
Unpaid loss and LAE, net of reinsurance recoverables	[1],[2]	2,617.1
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business | Accident Year 2011
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	966.4		972.9	976.0	959.8	$ 963.7	$ 973.0	$ 992.8	$ 1,007.6	$ 1,023.6	$ 1,031.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	10.5
Paid Loss and LAE, Net of Reinsurance	[2]	918.1		915.1	906.1	894.5	880.2	858.8	820.9	752.7	606.4	$ 376.5
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business | Accident Year 2012
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	933.9		941.4	948.4	958.9	983.9	1,005.5	1,056.8	1,059.9	1,022.3
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	12.4
Paid Loss and LAE, Net of Reinsurance	[2]	876.1		871.4	860.2	846.5	828.3	792.7	740.0	614.5	$ 354.8
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business | Accident Year 2013
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	769.6		780.7	790.3	804.1	823.4	838.4	868.8	850.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	18.7
Paid Loss and LAE, Net of Reinsurance	[2]	723.9		717.5	704.9	685.5	663.0	613.3	523.5	$ 265.4
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business | Accident Year 2014
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	752.5		755.0	772.4	784.3	808.8	833.0	850.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	30.9
Paid Loss and LAE, Net of Reinsurance	[2]	636.8		627.8	604.6	572.9	523.3	421.1	$ 251.2
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business | Accident Year 2015
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	776.2		776.7	798.4	784.0	768.6	763.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	30.7
Paid Loss and LAE, Net of Reinsurance	[2]	653.2		629.1	577.7	518.7	409.1	$ 218.5
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business | Accident Year 2016
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	1,092.6		1,100.3	1,111.6	1,124.4	1,118.0
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	41.9
Paid Loss and LAE, Net of Reinsurance	[2]	973.8		921.3	828.7	685.6	$ 367.2
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business | Accident Year 2017
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	994.5		1,026.9	1,046.3	1,095.9
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	81.1
Paid Loss and LAE, Net of Reinsurance	[2]	867.6		793.5	639.1	$ 342.8
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business | Accident Year 2018
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	1,270.7		1,266.1	1,255.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	148.9
Paid Loss and LAE, Net of Reinsurance	[2]	952.2		774.4	$ 397.9
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business | Accident Year 2019
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	1,411.8		1,408.6
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	280.8
Paid Loss and LAE, Net of Reinsurance	[2]	881.3		$ 462.0
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business | Accident Year 2020
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	1,408.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	799.3
Paid Loss and LAE, Net of Reinsurance	[2]	418.0
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business | Accident Year 2011 Through 2020
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	10,376.4
Paid Loss and LAE, Net of Reinsurance	[2]	$ 7,901.0

[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] Primarily represents the following reinsurance lines of business: ocean marine and aviation; auto liability; accident & health; mortgage reinsurance; surety; and credit.