Supplemental Information About Incurred and Paid Loss and LAE Development - Reinsurance Segment - Property (Detail) - USD ($) $ in Millions		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	Dec. 31, 2007
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables		$ 9,851.0	[1]	$ 9,629.9	$ 10,307.8	$ 10,650.4
Reinsurance Segment
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	7,944.4
Reinsurance Segment | Property
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2007		2.8
Unpaid loss and LAE, net of reinsurance recoverables	[1]	846.0
Reinsurance Segment | Property | Accident Year 2007
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		415.3		415.8	416.7	418.4	$ 419.6	$ 420.9	$ 423.8	$ 426.4	$ 437.5	$ 508.3
Paid Loss and LAE, Net of Reinsurance		411.0		411.0	410.0	409.8	407.5	402.7	393.8	362.8	306.2	$ 128.4
Reinsurance Segment | Property | Accident Year 2008
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		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		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		353.7		355.9	357.6	357.1	358.0	359.7	377.4	489.3
Paid Loss and LAE, Net of Reinsurance		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		609.9		606.0	581.2	540.4	528.0	527.1	615.6
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		25.4
Paid Loss and LAE, Net of Reinsurance		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		1,217.1		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		9.6
Paid Loss and LAE, Net of Reinsurance		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		478.1		495.6	530.3	579.1	697.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		21.3
Paid Loss and LAE, Net of Reinsurance		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		422.1		444.6	470.6	501.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		21.1
Paid Loss and LAE, Net of Reinsurance		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		451.3		464.8	496.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		37.4
Paid Loss and LAE, Net of Reinsurance		360.0		297.6	$ 109.4
Reinsurance Segment | Property | Accident Year 2015
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		332.0		368.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		59.7
Paid Loss and LAE, Net of Reinsurance		217.7		$ 96.0
Reinsurance Segment | Property | Accident Year 2016
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		684.1
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		262.2
Paid Loss and LAE, Net of Reinsurance		210.5
Reinsurance Segment | Property | Accident Year 2007 through 2016
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		5,472.4
Paid Loss and LAE, Net of Reinsurance		$ 4,629.2

[1] Includes unallocated LAE, which aggregate to 1.3 percent of gross loss and LAE reserves as of December 31, 2016. 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.