Information About Incurred and Paid Loss and LAE Development - Reinsurance Segment - Property (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 | Property
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2009		3.5
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,400.7
Reinsurance Segment | Property | Accident Year 2009
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		352.6		353.6	353.7	355.9	$ 357.6	$ 357.1	$ 358.0	$ 359.7	$ 377.4	$ 489.3
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		0.0
Paid Loss and LAE, Net of Reinsurance		345.8		347.1	345.7	347.9	345.5	340.6	332.4	310.9	251.7	$ 114.7
Reinsurance Segment | Property | Accident Year 2010
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		605.3		613.0	609.9	606.0	581.2	540.4	528.0	527.1	615.6
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		16.6
Paid Loss and LAE, Net of Reinsurance		575.1		571.5	563.7	540.1	513.5	472.3	418.9	349.0	$ 169.3
Reinsurance Segment | Property | Accident Year 2011
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		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		1.3
Paid Loss and LAE, Net of Reinsurance		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		462.5		469.4	478.1	495.6	530.3	579.1	697.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		7.0
Paid Loss and LAE, Net of Reinsurance		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		403.6		406.8	422.1	444.6	470.6	501.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		7.0
Paid Loss and LAE, Net of Reinsurance		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		429.8		435.2	451.3	464.8	496.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		12.8
Paid Loss and LAE, Net of Reinsurance		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		306.3		320.6	332.0	368.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		15.2
Paid Loss and LAE, Net of Reinsurance		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		596.7		647.2	684.1
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		32.7
Paid Loss and LAE, Net of Reinsurance		504.8		429.8	$ 210.5
Reinsurance Segment | Property | Accident Year 2017
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		1,179.5		1,174.6
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		76.3
Paid Loss and LAE, Net of Reinsurance		818.5		$ 324.5
Reinsurance Segment | Property | Accident Year 2018
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		1,152.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		538.4
Paid Loss and LAE, Net of Reinsurance		309.5
Reinsurance Segment | Property | Accident Year 2009 Through 2018
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		6,698.6
Paid Loss and LAE, Net of Reinsurance		$ 5,301.4

[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.