Information About Incurred and Paid Loss and LAE Development - Reinsurance Segment - Casualty & Other - Longer-Tailed Lines of Business (Detail) - USD ($) $ in Millions		Dec. 31, 2018		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
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
Reinsurance Segment
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	8,478.5
Reinsurance Segment | Casualty & Other | Longer Tailed Lines of Business
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2009		755.2
Unpaid loss and LAE, net of reinsurance recoverables	[1],[2]	4,682.6
Reinsurance Segment | Casualty & Other | Longer Tailed Lines of Business | Accident Year 2009
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		1,108.1		1,119.8	1,123.4	1,129.4	$ 1,107.8	$ 1,094.9	$ 1,101.9	$ 1,091.0	$ 1,074.7	$ 1,059.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		90.4
Paid Loss and LAE, Net of Reinsurance		904.1		859.8	814.4	749.2	621.8	493.3	366.7	221.2	105.0	$ 24.8
Reinsurance Segment | Casualty & Other | Longer Tailed Lines of Business | Accident Year 2010
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		959.7		962.9	977.6	999.3	1,015.5	1,063.5	1,090.7	1,101.9	1,098.6
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		133.4
Paid Loss and LAE, Net of Reinsurance		735.8		688.9	644.3	573.9	438.7	322.3	208.5	108.4	$ 33.8
Reinsurance Segment | Casualty & Other | Longer Tailed Lines of Business | Accident Year 2011
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		925.5		942.6	949.9	959.1	974.6	994.6	1,009.6	996.0
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		152.4
Paid Loss and LAE, Net of Reinsurance		709.1		663.0	595.1	501.8	373.4	206.4	89.4	$ 29.6
Reinsurance Segment | Casualty & Other | Longer Tailed Lines of Business | Accident Year 2012
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		832.4		829.8	833.1	860.8	875.9	877.5	877.5
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		168.5
Paid Loss and LAE, Net of Reinsurance		587.0		521.3	435.7	316.6	201.1	106.8	$ 46.9
Reinsurance Segment | Casualty & Other | Longer Tailed Lines of Business | Accident Year 2013
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		783.0		776.7	777.5	799.2	802.2	798.6
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		234.3
Paid Loss and LAE, Net of Reinsurance		452.7		378.0	296.4	176.6	85.1	$ 22.2
Reinsurance Segment | Casualty & Other | Longer Tailed Lines of Business | Accident Year 2014
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		790.3		793.9	794.3	791.1	801.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		273.8
Paid Loss and LAE, Net of Reinsurance		444.7		342.9	229.6	107.5	$ 29.8
Reinsurance Segment | Casualty & Other | Longer Tailed Lines of Business | Accident Year 2015
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		793.7		797.0	799.0	794.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		321.9
Paid Loss and LAE, Net of Reinsurance		325.8		205.0	100.4	$ 32.2
Reinsurance Segment | Casualty & Other | Longer Tailed Lines of Business | Accident Year 2016
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		787.4		776.8	776.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		399.5
Paid Loss and LAE, Net of Reinsurance		242.9		121.8	$ 34.8
Reinsurance Segment | Casualty & Other | Longer Tailed Lines of Business | Accident Year 2017
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		765.7		764.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		543.0
Paid Loss and LAE, Net of Reinsurance		110.6		$ 34.0
Reinsurance Segment | Casualty & Other | Longer Tailed Lines of Business | Accident Year 2018
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		727.3
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		650.8
Paid Loss and LAE, Net of Reinsurance		33.0
Reinsurance Segment | Casualty & Other | Longer Tailed Lines of Business | Accident Year 2009 Through 2018
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		8,473.1
Paid Loss and LAE, Net of Reinsurance		$ 4,545.7

[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 the following reinsurance lines of business: directors’ and officers’ liability; errors and omissions liability; general liability; and medical malpractice.