Information About Incurred and Paid Loss and LAE Development - Reinsurance Segment - Casualty & Other - Longer-Tailed Lines of Business (Detail) - USD ($) $ in Millions		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	Dec. 31, 2008
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
Reinsurance Segment
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables	[1]	8,383.3
Reinsurance Segment | Casualty & Other | Longer Tailed Lines of Business
Claims Development [Line Items]
Unpaid loss and LAE, net of reinsurance recoverables, prior to 2008		689.7
Unpaid loss and LAE, net of reinsurance recoverables	[1]	4,874.2
Reinsurance Segment | Casualty & Other | Longer Tailed Lines of Business | Accident Year 2008
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		1,173.1		1,178.7	1,187.2	1,209.8	$ 1,205.4	$ 1,199.2	$ 1,180.4	$ 1,150.6	$ 1,126.5	$ 1,105.1
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		119.3
Paid Loss and LAE, Net of Reinsurance		937.8		901.2	852.0	766.5	655.4	521.1	374.8	214.5	99.5	$ 55.2
Reinsurance Segment | Casualty & Other | Longer Tailed Lines of Business | Accident Year 2009
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		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		119.0
Paid Loss and LAE, Net of Reinsurance		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		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		175.6
Paid Loss and LAE, Net of Reinsurance		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		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		189.3
Paid Loss and LAE, Net of Reinsurance		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		829.8		833.1	860.8	875.9	877.5	877.5
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		208.1
Paid Loss and LAE, Net of Reinsurance		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		776.7		777.5	799.2	802.2	798.6
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		288.3
Paid Loss and LAE, Net of Reinsurance		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		793.9		794.3	791.1	801.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		365.9
Paid Loss and LAE, Net of Reinsurance		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		797.0		799.0	794.8
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		455.8
Paid Loss and LAE, Net of Reinsurance		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		776.8		776.7
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		547.0
Paid Loss and LAE, Net of Reinsurance		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		764.4
Incurred But Not Reported Loss and Loss Adjustment Expense Reserves		681.6
Paid Loss and LAE, Net of Reinsurance		34.0
Reinsurance Segment | Casualty & Other | Longer Tailed Lines of Business | Accident Year 2008 Through 2017
Claims Development [Line Items]
Incurred Loss and LAE, net of reinsurance		8,937.0
Paid Loss and LAE, Net of Reinsurance		$ 4,752.5

[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.