Information About Incurred and Paid Loss and LAE Development - Reinsurance Segment - Casualty & Other - Shorter-Tailed Lines of Business (Detail) - USD ($) $ in Millions		Dec. 31, 2018		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
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
Reinsurance Segment
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	8,478.5
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2009		155.9
Unpaid loss and LAE, net of reinsurance recoverables	[1]	2,395.2
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business | Accident Year 2009
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	1,069.9		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	[2]	10.2
Paid Loss and LAE, Net of Reinsurance		1,028.3		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	[2]	927.7		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	[2]	13.7
Paid Loss and LAE, Net of Reinsurance		909.6		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	[2]	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]	24.7
Paid Loss and LAE, Net of Reinsurance		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]	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]	30.8
Paid Loss and LAE, Net of Reinsurance		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]	790.3		804.1	823.4	838.4	868.8	850.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	44.0
Paid Loss and LAE, Net of Reinsurance		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]	772.4		784.3	808.8	833.0	850.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	63.9
Paid Loss and LAE, Net of Reinsurance		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]	798.4		784.0	768.6	763.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	73.5
Paid Loss and LAE, Net of Reinsurance		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]	1,111.6		1,124.4	1,118.0
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	139.5
Paid Loss and LAE, Net of Reinsurance		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]	1,046.3		1,095.9
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	491.5
Paid Loss and LAE, Net of Reinsurance		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]	1,255.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	627.7
Paid Loss and LAE, Net of Reinsurance		397.9
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business | Accident Year 2009 Through 2018
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	9,696.4
Paid Loss and LAE, Net of Reinsurance		$ 7,457.1

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