Information About Incurred and Paid Loss and LAE Development - Reinsurance Segment - Casualty & Other - Shorter-Tailed Lines of Business (Detail) - USD ($) $ in Millions		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables		$ 10,221.2	[1]	$ 9,851.0	$ 9,629.9	$ 10,307.8
Reinsurance Segment
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	8,383.3
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2008		163.9
Unpaid loss and LAE, net of reinsurance recoverables	[1]	2,244.8
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business | Accident Year 2008
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		1,112.7		1,114.9	1,117.2	1,115.0	$ 1,117.9	$ 1,114.1	$ 1,114.0	$ 1,161.2	$ 1,174.6	$ 1,157.6
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		9.8
Paid Loss and LAE, Net of Reinsurance		1,077.4		1,071.3	1,060.0	1,042.7	1,020.8	994.2	962.4	922.6	753.8	$ 397.5
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business | Accident Year 2009
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		1,062.9		1,062.7	1,064.1	1,063.8	1,071.0	1,066.8	1,086.5	1,114.0	1,169.1
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		13.2
Paid Loss and LAE, Net of Reinsurance		1,021.9		1,014.3	1,000.0	975.5	951.9	918.7	868.6	745.2	$ 457.2
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business | Accident Year 2010
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		943.4		950.3	954.0	967.2	978.3	985.9	1,007.0	1,024.5
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		17.9
Paid Loss and LAE, Net of Reinsurance		903.7		897.5	884.8	866.7	832.7	776.5	671.3	$ 404.9
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business | Accident Year 2011
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		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		28.9
Paid Loss and LAE, Net of Reinsurance		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		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		45.7
Paid Loss and LAE, Net of Reinsurance		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		804.1		823.4	838.4	868.8	850.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		53.6
Paid Loss and LAE, Net of Reinsurance		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		784.3		808.8	833.0	850.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		89.8
Paid Loss and LAE, Net of Reinsurance		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		784.0		768.6	763.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		110.1
Paid Loss and LAE, Net of Reinsurance		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		1,124.4		1,118.0
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		247.0
Paid Loss and LAE, Net of Reinsurance		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		1,095.9
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		626.4
Paid Loss and LAE, Net of Reinsurance		342.8
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business | Accident Year 2008 Through 2017
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		9,630.4
Paid Loss and LAE, Net of Reinsurance		$ 7,549.5

[1] Includes unallocated LAE, which aggregate to 1.3 percent of gross loss and LAE reserves as of December 31, 2017. 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.