Supplemental Information About Incurred and Paid Loss and LAE Development - Insurance Segment - RSUI - Casualty (Detail) $ in Millions		Dec. 31, 2016 USD ($) Claim		Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)	Dec. 31, 2007 USD ($)
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
Insurance Segment
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,906.6
Insurance Segment | RSUI
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,437.5
Insurance Segment | RSUI | Casualty Insurance
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2007		54.0
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,229.0
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2007
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		201.7		206.4	215.5	225.7	$ 232.6	$ 243.9	$ 252.2	$ 253.6	$ 257.6	$ 252.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 22.1
Cumulative Number of Reported Claims | Claim	[2]	5,992
Paid Loss and LAE, Net of Reinsurance		$ 171.9		170.5	164.2	158.8	151.4	117.4	88.9	63.0	24.2	$ 4.1
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2008
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		216.9		220.9	230.8	237.0	245.3	254.8	260.6	255.0	255.0
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 27.5
Cumulative Number of Reported Claims | Claim	[2]	7,105
Paid Loss and LAE, Net of Reinsurance		$ 179.4		171.3	164.2	157.4	145.3	124.4	89.5	35.1	$ 9.5
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2009
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		212.0		221.8	232.2	236.3	236.3	234.4	233.7	230.1
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 34.8
Cumulative Number of Reported Claims | Claim	[2]	6,843
Paid Loss and LAE, Net of Reinsurance		$ 166.4		157.5	149.5	136.9	101.9	73.2	38.0	$ 7.2
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2010
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		177.0		178.6	194.6	203.4	204.1	204.1	204.1
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 28.9
Cumulative Number of Reported Claims | Claim	[2]	6,859
Paid Loss and LAE, Net of Reinsurance		$ 132.9		128.6	122.6	90.1	70.1	30.9	$ 4.7
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2011
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		206.8		211.6	212.1	208.3	205.9	205.9
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 45.3
Cumulative Number of Reported Claims | Claim	[2]	7,459
Paid Loss and LAE, Net of Reinsurance		$ 138.5		118.4	100.3	66.7	31.9	$ 6.5
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2012
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		238.9		242.8	230.3	226.3	226.3
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 72.6
Cumulative Number of Reported Claims | Claim	[2]	7,518
Paid Loss and LAE, Net of Reinsurance		$ 144.0		125.5	96.0	38.4	$ 6.8
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2013
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		280.1		277.6	264.8	264.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 99.4
Cumulative Number of Reported Claims | Claim	[2]	8,503
Paid Loss and LAE, Net of Reinsurance		$ 146.2		103.4	50.0	$ 10.1
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2014
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		321.1		322.7	292.0
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 144.3
Cumulative Number of Reported Claims | Claim	[2]	9,870
Paid Loss and LAE, Net of Reinsurance		$ 130.1		69.5	$ 13.0
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2015
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		300.2		300.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 207.6
Cumulative Number of Reported Claims | Claim	[2]	8,661
Paid Loss and LAE, Net of Reinsurance		$ 47.3		$ 9.0
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2016
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		290.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 252.2
Cumulative Number of Reported Claims | Claim	[2]	7,156
Paid Loss and LAE, Net of Reinsurance		$ 13.7
Insurance Segment | RSUI | Casualty Insurance | Accident Year 2007 through 2016
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		2,445.4
Paid Loss and LAE, Net of Reinsurance		$ 1,270.4

[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.

[2] Represents claims reported by insured claimants.