Property and Casualty Insurance Reserves - Short-duration Insurance Contracts, Claims Development (Details) $ in Millions	Dec. 31, 2024 USD ($) claim	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Loss and Allocated LAE Reserves, Net of Reinsurance	$ 2,399.4
Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	7,775.6
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	6,364.6
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	38.7
Loss and Allocated LAE Reserves, Net of Reinsurance	1,449.7
Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	3,833.7
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	3,804.5
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	(2.8)
Loss and Allocated LAE Reserves, Net of Reinsurance	26.4
Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,496.3
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	833.6
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	9.7
Loss and Allocated LAE Reserves, Net of Reinsurance	672.4
Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	328.0
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	310.8
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	(0.1)
Loss and Allocated LAE Reserves, Net of Reinsurance	17.1
Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	710.1
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	555.0
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	4.5
Loss and Allocated LAE Reserves, Net of Reinsurance	159.6
Preferred Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	444.6
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	445.8
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	0.0
Loss and Allocated LAE Reserves, Net of Reinsurance	(1.2)
Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	657.2
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	618.4
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	0.8
Loss and Allocated LAE Reserves, Net of Reinsurance	39.6
2020 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,417.8	$ 1,415.9	$ 1,407.8	$ 1,406.4	$ 1,401.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 20.1
Cumulative Number of Reported Claims | claim	476,106
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 1,381.3	1,350.0	1,287.8	1,107.6	555.2
2020 | Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	659.7	659.0	659.5	659.5	650.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.1)
Cumulative Number of Reported Claims | claim	296,478
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 659.4	658.8	659.7	663.8	585.5
2020 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	159.7	155.6	154.0	152.0	140.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 4.1
Cumulative Number of Reported Claims | claim	19,662
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 144.5	129.7	111.7	87.6	37.0
2020 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	32.3	32.1	32.1	32.2	31.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.1
Cumulative Number of Reported Claims | claim	11,041
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 32.1	32.0	32.0	31.9	26.2
2020 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	158.4	158.8	151.8	153.6	148.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 1.3
Cumulative Number of Reported Claims | claim	24,701
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 149.8	141.4	117.7	92.8	44.4
2020 | Preferred Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	97.5	97.5	97.9	98.0	96.1
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.0
Cumulative Number of Reported Claims | claim	47,591
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 97.5	97.5	97.6	98.4	90.9
2020 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	141.8	141.2	144.6	149.8	157.0
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.2
Cumulative Number of Reported Claims | claim	14,102
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 140.8	139.8	137.4	130.8	$ 94.6
2021 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,861.0	1,844.2	1,824.7	1,856.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 52.8
Cumulative Number of Reported Claims | claim	586,566
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 1,767.5	1,680.8	1,429.4	657.1
2021 | Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	967.4	967.2	967.5	958.0
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.1)
Cumulative Number of Reported Claims | claim	361,993
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 967.2	968.3	977.5	890.1
2021 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	250.4	240.4	228.6	225.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 13.9
Cumulative Number of Reported Claims | claim	27,486
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 208.3	168.6	128.0	50.8
2021 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	51.8	51.6	51.9	52.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.3
Cumulative Number of Reported Claims | claim	17,703
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 51.5	51.4	51.9	43.3
2021 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	182.0	180.6	179.8	176.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 3.7
Cumulative Number of Reported Claims | claim	27,243
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 161.9	144.1	106.1	50.3
2021 | Preferred Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	117.1	117.1	117.9	118.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.0
Cumulative Number of Reported Claims | claim	53,490
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 117.1	117.2	118.1	113.1
2021 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	144.3	143.9	149.8	149.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.5
Cumulative Number of Reported Claims | claim	13,555
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 141.3	139.7	132.6	$ 100.6
2022 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,880.7	1,848.7	1,765.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 98.0
Cumulative Number of Reported Claims | claim	475,177
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 1,700.4	1,463.3	738.2
2022 | Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	990.0	989.5	993.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (1.7)
Cumulative Number of Reported Claims | claim	309,091
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 990.5	997.8	921.9
2022 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	317.9	309.1	305.1
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 34.8
Cumulative Number of Reported Claims | claim	32,476
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 222.1	159.0	72.2
2022 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	74.8	74.7	74.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.5
Cumulative Number of Reported Claims | claim	21,567
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 74.2	74.6	66.8
2022 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	173.5	172.4	165.0
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 8.6
Cumulative Number of Reported Claims | claim	24,246
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 139.1	111.0	55.0
2022 | Preferred Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	113.6	113.5	110.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.2)
Cumulative Number of Reported Claims | claim	48,160
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 113.7	114.6	108.7
2022 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	155.2	152.7	142.7
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 1.1
Cumulative Number of Reported Claims | claim	11,526
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 149.3	141.2	$ 97.0
2023 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,391.7	1,448.7
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 150.5
Cumulative Number of Reported Claims | claim	304,878
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 1,092.8	580.4
2023 | Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	715.8	722.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (5.9)
Cumulative Number of Reported Claims | claim	206,366
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 720.7	699.2
2023 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	361.7	379.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 87.1
Cumulative Number of Reported Claims | claim	35,327
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 189.6	87.5
2023 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	87.5	90.0
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.2
Cumulative Number of Reported Claims | claim	20,202
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 86.9	80.6
2023 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	137.5	135.0
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 17.6
Cumulative Number of Reported Claims | claim	16,796
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 84.6	43.7
2023 | Preferred Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	84.7	86.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.7)
Cumulative Number of Reported Claims | claim	33,951
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 85.4	84.8
2023 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	135.3	126.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 1.6
Cumulative Number of Reported Claims | claim	9,919
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 127.8	$ 84.7
2024 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,224.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 506.0
Cumulative Number of Reported Claims | claim	216,146
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 422.6
2024 | Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	500.8
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 7.1
Cumulative Number of Reported Claims | claim	136,921
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 466.7
2024 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	406.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 249.0
Cumulative Number of Reported Claims | claim	29,666
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 69.1
2024 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	81.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 7.7
Cumulative Number of Reported Claims | claim	16,312
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 66.1
2024 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	58.7
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 19.8
Cumulative Number of Reported Claims | claim	6,666
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 19.6
2024 | Preferred Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	31.7
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (1.2)
Cumulative Number of Reported Claims | claim	12,556
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 32.1
2024 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	80.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 7.2
Cumulative Number of Reported Claims | claim	4,153
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 59.2