Information About Incurred and Paid Loss and LAE Development - Reinsurance Segment - Property (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 | Property
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2008		3.8
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,264.3
Reinsurance Segment | Property | Accident Year 2008
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		506.3		508.8	508.7	509.4	$ 510.9	$ 511.9	$ 515.9	$ 510.9	$ 508.0	$ 645.0
Paid Loss and LAE, Net of Reinsurance		503.3		505.6	505.0	503.1	501.5	495.0	477.1	435.2	350.8	$ 163.3
Reinsurance Segment | Property | Accident Year 2009
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		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		0.3
Paid Loss and LAE, Net of Reinsurance		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		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		22.8
Paid Loss and LAE, Net of Reinsurance		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		1,210.9		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		3.2
Paid Loss and LAE, Net of Reinsurance		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		469.4		478.1	495.6	530.3	579.1	697.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		13.2
Paid Loss and LAE, Net of Reinsurance		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		406.8		422.1	444.6	470.6	501.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		11.8
Paid Loss and LAE, Net of Reinsurance		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		435.2		451.3	464.8	496.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		20.2
Paid Loss and LAE, Net of Reinsurance		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		320.6		332.0	368.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		31.9
Paid Loss and LAE, Net of Reinsurance		278.3		217.7	$ 96.0
Reinsurance Segment | Property | Accident Year 2016
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		647.2		684.1
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		92.4
Paid Loss and LAE, Net of Reinsurance		429.8		$ 210.5
Reinsurance Segment | Property | Accident Year 2017
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		1,174.6
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		413.9
Paid Loss and LAE, Net of Reinsurance		324.5
Reinsurance Segment | Property | Accident Year 2008 Through 2017
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		6,137.6
Paid Loss and LAE, Net of Reinsurance		$ 4,877.1

[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.