Information About Incurred and Paid Loss and LAE Development - Insurance Segment - CapSpecialty (Detail) $ in Millions		Dec. 31, 2018 USD ($) Claim		Dec. 31, 2017 USD ($)	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 ($)	Dec. 31, 2009 USD ($)
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
Insurance Segment
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	1,914.4
Insurance Segment | CapSpecialty Incorporated
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2009		8.8
Unpaid loss and LAE, net of reinsurance recoverables	[1]	307.6
Insurance Segment | CapSpecialty Incorporated | Accident Year 2009
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		97.8		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		$ 0.3
Cumulative Number of Reported Claims | Claim	[2]	8,542
Paid Loss and LAE, Net of Reinsurance		$ 96.5		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		127.2		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		$ 1.8
Cumulative Number of Reported Claims | Claim	[2]	8,211
Paid Loss and LAE, Net of Reinsurance		$ 124.4		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		77.7		78.2	78.4	79.4	76.9	74.4	71.2	74.0
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 0.7
Cumulative Number of Reported Claims | Claim	[2]	5,688
Paid Loss and LAE, Net of Reinsurance		$ 75.7		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		68.8		68.7	69.7	69.3	66.2	71.8	72.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 0.7
Cumulative Number of Reported Claims | Claim	[2]	5,262
Paid Loss and LAE, Net of Reinsurance		$ 66.5		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		88.6		87.2	84.4	85.2	81.4	78.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 1.8
Cumulative Number of Reported Claims | Claim	[2]	5,158
Paid Loss and LAE, Net of Reinsurance		$ 84.5		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		98.6		100.2	101.0	102.7	102.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 4.6
Cumulative Number of Reported Claims | Claim	[2]	5,896
Paid Loss and LAE, Net of Reinsurance		$ 89.8		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		109.7		109.2	111.8	111.0
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 8.3
Cumulative Number of Reported Claims | Claim	[2]	5,456
Paid Loss and LAE, Net of Reinsurance		$ 88.8		73.5	57.4	$ 30.9
Insurance Segment | CapSpecialty Incorporated | Accident Year 2016
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		132.7		132.4	129.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 23.5
Cumulative Number of Reported Claims | Claim	[2]	5,993
Paid Loss and LAE, Net of Reinsurance		$ 86.6		64.0	$ 30.3
Insurance Segment | CapSpecialty Incorporated | Accident Year 2017
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		143.7		147.3
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 47.5
Cumulative Number of Reported Claims | Claim	[2]	6,046
Paid Loss and LAE, Net of Reinsurance		$ 65.6		$ 30.1
Insurance Segment | CapSpecialty Incorporated | Accident Year 2018
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		164.1
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		$ 99.3
Cumulative Number of Reported Claims | Claim	[2]	5,428
Paid Loss and LAE, Net of Reinsurance		$ 31.7
Insurance Segment | CapSpecialty Incorporated | Accident Year 2009 Through 2018
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		1,108.9
Paid Loss and LAE, Net of Reinsurance		$ 810.1

[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.

[2] Represents claims reported by insured claimants.