Loss And Loss Adjustment Expense Reserves Loss And Loss Adjustment Expense Reserves - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2022 USD ($) data_subset	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Number of subsets of business data reviewed (over) | data_subset	400
Reserve development, prior years	$ (86.3)	$ (4.7)	$ 195.3
2021
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years	(22.0)
2020
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years	(47.0)	(140.0)
2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years		87.0	59.0
2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years			107.0
Commercial Lines Segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years	$ 82.0	87.0	98.0
Property Segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years		36.0	(14.0)
Environmental, asbestos, and general liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Percentage of reserves	2.00%
Auto
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Percentage of reserves	90.00%
Personal Auto | Personal Lines Segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years	$ (157.0)	(113.0)	136.0
Special Lines | Personal Lines Segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years		(14.0)	$ (25.0)
Direct Channel | Personal Auto | Personal Lines Segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years		$ (80.0)