Information About Incurred and Paid Loss and LAE Development - Insurance Segment - CapSpecialty (Detail) $ in Millions		Dec. 31, 2017 USD ($) Claim		Dec. 31, 2016 USD ($)	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 ($)	Dec. 31, 2008 USD ($)
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
Insurance Segment
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,837.9
Insurance Segment | CapSpecialty Incorporated
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2008	[2]	9.2
Unpaid loss and LAE, net of reinsurance recoverables	[1],[2]	270.5
Insurance Segment | CapSpecialty Incorporated | Accident Year 2008
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	94.6		96.8	96.5	94.2	$ 94.3	$ 89.6	$ 90.1	$ 93.2	$ 97.1	$ 102.6
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	$ 0.4
Cumulative Number of Reported Claims | Claim	[3]	8,765
Paid Loss and LAE, Net of Reinsurance	[2]	$ 93.7		93.1	91.7	90.3	87.2	82.4	74.6	63.9	43.8	$ 26.3
Insurance Segment | CapSpecialty Incorporated | Accident Year 2009
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	97.9		98.0	97.9	99.0	98.8	95.7	90.4	93.3	92.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	$ 0.5
Cumulative Number of Reported Claims | Claim	[3]	8,330
Paid Loss and LAE, Net of Reinsurance	[2]	$ 96.4		95.7	94.4	92.7	87.2	78.2	62.5	50.1	$ 27.7
Insurance Segment | CapSpecialty Incorporated | Accident Year 2010
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	127.7		129.5	129.8	134.5	134.8	117.9	110.8	93.2
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	$ 2.4
Cumulative Number of Reported Claims | Claim	[3]	8,170
Paid Loss and LAE, Net of Reinsurance	[2]	$ 123.9		121.6	119.2	111.0	98.1	78.4	56.2	$ 22.0
Insurance Segment | CapSpecialty Incorporated | Accident Year 2011
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	78.2		78.4	79.4	76.9	74.4	71.2	74.0
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	$ 1.8
Cumulative Number of Reported Claims | Claim	[3]	5,655
Paid Loss and LAE, Net of Reinsurance	[2]	$ 73.8		69.8	67.3	57.7	44.7	31.9	$ 16.3
Insurance Segment | CapSpecialty Incorporated | Accident Year 2012
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	68.7		69.7	69.3	66.2	71.8	72.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	$ 0.9
Cumulative Number of Reported Claims | Claim	[3]	5,233
Paid Loss and LAE, Net of Reinsurance	[2]	$ 65.0		63.3	57.2	46.9	38.6	$ 18.6
Insurance Segment | CapSpecialty Incorporated | Accident Year 2013
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	87.2		84.4	85.2	81.4	78.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	$ 3.0
Cumulative Number of Reported Claims | Claim	[3]	5,103
Paid Loss and LAE, Net of Reinsurance	[2]	$ 78.6		69.6	62.0	48.0	$ 23.4
Insurance Segment | CapSpecialty Incorporated | Accident Year 2014
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	100.2		101.0	102.7	102.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	$ 8.5
Cumulative Number of Reported Claims | Claim	[3]	5,810
Paid Loss and LAE, Net of Reinsurance	[2]	$ 83.1		71.9	56.3	$ 34.0
Insurance Segment | CapSpecialty Incorporated | Accident Year 2015
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	109.2		111.8	111.0
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	$ 19.3
Cumulative Number of Reported Claims | Claim	[3]	5,344
Paid Loss and LAE, Net of Reinsurance	[2]	$ 73.5		57.4	$ 30.9
Insurance Segment | CapSpecialty Incorporated | Accident Year 2016
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	132.4		129.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	$ 45.5
Cumulative Number of Reported Claims | Claim	[3]	5,743
Paid Loss and LAE, Net of Reinsurance	[2]	$ 64.0		$ 30.3
Insurance Segment | CapSpecialty Incorporated | Accident Year 2017
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	147.3
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves	[2]	$ 89.9
Cumulative Number of Reported Claims | Claim	[3]	4,924
Paid Loss and LAE, Net of Reinsurance	[2]	$ 30.1
Insurance Segment | CapSpecialty Incorporated | Accident Year 2008 Through 2017
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance	[2]	1,043.4
Paid Loss and LAE, Net of Reinsurance	[2]	$ 782.1

[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.

[2] The vast majority of the CapSpecialty's loss and LAE reserves relate to its casualty lines of business.

[3] Represents claims reported by insured claimants.