Property and Casualty Insurance Reserves - Short-duration Insurance Contracts, Claims Development (Details) $ in Millions	Dec. 31, 2023 USD ($) claim	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Loss and Allocated LAE Reserves, Net of Reinsurance	$ 2,450.8
Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	8,073.6
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	6,556.6
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	27.9
Loss and Allocated LAE Reserves, Net of Reinsurance	1,544.9
Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	3,967.8
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	3,953.9
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	(2.8)
Loss and Allocated LAE Reserves, Net of Reinsurance	11.1
Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,214.8
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	666.7
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	11.4
Loss and Allocated LAE Reserves, Net of Reinsurance	559.5
Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	275.2
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	265.5
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	0.0
Loss and Allocated LAE Reserves, Net of Reinsurance	9.7
Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	851.6
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	633.7
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	8.2
Loss and Allocated LAE Reserves, Net of Reinsurance	226.1
Preferred Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	540.1
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	539.5
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	0.0
Loss and Allocated LAE Reserves, Net of Reinsurance	0.6
Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	726.0
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	666.0
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance	1.5
Loss and Allocated LAE Reserves, Net of Reinsurance	61.5
2015 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,516.1	$ 1,508.3	$ 1,506.1	$ 1,494.7	$ 1,461.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 20.0
Cumulative Number of Reported Claims | claim	547,437
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 1,482.1	1,452.3	1,382.0	1,200.7	567.3
2015 | Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	629.5	629.7	629.6	630.3	624.3
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (9.2)
Cumulative Number of Reported Claims | claim	324,516
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 629.8	630.0	630.6	634.8	570.8
2015 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	129.8	128.1	126.6	126.1	128.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 5.7
Cumulative Number of Reported Claims | claim	19,641
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 121.9	113.1	99.5	75.7	32.4
2015 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	26.8	26.8	26.9	27.1	26.0
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.5)
Cumulative Number of Reported Claims | claim	9,317
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 26.9	26.8	26.8	26.9	23.0
2015 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	204.8	201.8	200.0	195.5	172.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 1.6
Cumulative Number of Reported Claims | claim	34,622
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 193.5	181.1	160.8	127.9	62.7
2015 | Preferred Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	125.4	125.8	125.9	125.8	126.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.0
Cumulative Number of Reported Claims | claim	67,108
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 125.4	125.4	125.6	126.5	120.7
2015 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	161.6	162.8	163.1	161.8	162.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.4
Cumulative Number of Reported Claims | claim	14,531
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 160.6	159.5	157.7	150.4	$ 111.1
2016 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,415.9	1,407.8	1,406.4	1,401.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 29.9
Cumulative Number of Reported Claims | claim	475,894
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 1,350.0	1,287.8	1,107.6	555.2
2016 | Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	659.0	659.5	659.5	650.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.5)
Cumulative Number of Reported Claims | claim	296,411
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 658.8	659.7	663.8	585.5
2016 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	155.6	154.0	152.0	140.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 7.4
Cumulative Number of Reported Claims | claim	19,627
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 129.7	111.7	87.6	37.0
2016 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	32.1	32.1	32.2	31.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.0
Cumulative Number of Reported Claims | claim	11,044
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 32.0	32.0	31.9	26.2
2016 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	158.8	151.8	153.6	148.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 3.1
Cumulative Number of Reported Claims | claim	24,664
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 141.4	117.7	92.8	44.4
2016 | 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	97.9	98.0	96.1
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.0
Cumulative Number of Reported Claims | claim	47,589
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 97.5	97.6	98.4	90.9
2016 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	141.2	144.6	149.8	157.0
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.4
Cumulative Number of Reported Claims | claim	14,074
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 139.8	137.4	130.8	$ 94.6
2017 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,844.2	1,824.7	1,856.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 75.9
Cumulative Number of Reported Claims | claim	585,665
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 1,680.8	1,429.4	657.1
2017 | Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	967.2	967.5	958.0
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (1.9)
Cumulative Number of Reported Claims | claim	361,864
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 968.3	977.5	890.1
2017 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	240.4	228.6	225.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 24.8
Cumulative Number of Reported Claims | claim	27,382
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 168.6	128.0	50.8
2017 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	51.6	51.9	52.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.2
Cumulative Number of Reported Claims | claim	17,724
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 51.4	51.9	43.3
2017 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	180.6	179.8	176.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 9.3
Cumulative Number of Reported Claims | claim	27,173
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 144.1	106.1	50.3
2017 | 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	117.9	118.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.2)
Cumulative Number of Reported Claims | claim	53,477
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 117.2	118.1	113.1
2017 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	143.9	149.8	149.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0.9
Cumulative Number of Reported Claims | claim	13,620
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 139.7	132.6	$ 100.6
2018 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,848.7	1,765.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 169.3
Cumulative Number of Reported Claims | claim	471,990
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 1,463.3	738.2
2018 | Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	989.5	993.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (9.7)
Cumulative Number of Reported Claims | claim	308,705
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 997.8	921.9
2018 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	309.1	305.1
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 67.1
Cumulative Number of Reported Claims | claim	32,102
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 159.0	72.2
2018 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	74.7	74.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.2)
Cumulative Number of Reported Claims | claim	21,541
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 74.6	66.8
2018 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	172.4	165.0
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 19.9
Cumulative Number of Reported Claims | claim	24,019
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 111.0	55.0
2018 | Preferred Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	113.5	110.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (1.1)
Cumulative Number of Reported Claims | claim	48,108
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 114.6	108.7
2018 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	152.7	142.7
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 1.7
Cumulative Number of Reported Claims | claim	11,463
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 141.2	$ 97.0
2019 | Specialty Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	1,448.7
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 537.1
Cumulative Number of Reported Claims | claim	282,423
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 580.4
2019 | Specialty Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	722.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (6.2)
Cumulative Number of Reported Claims | claim	199,413
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 699.2
2019 | Commercial Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	379.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 201.4
Cumulative Number of Reported Claims | claim	31,970
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 87.5
2019 | Commercial Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	90.0
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 4.8
Cumulative Number of Reported Claims | claim	19,056
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 80.6
2019 | Preferred Personal Automobile Insurance—Liability
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	135.0
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 47.1
Cumulative Number of Reported Claims | claim	15,795
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 43.7
2019 | Preferred Personal Automobile Insurance—Physical Damage
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	86.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (1.3)
Cumulative Number of Reported Claims | claim	32,552
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 84.8
2019 | Homeowners Insurance
Claims Development [Line Items]
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31,	126.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 12.8
Cumulative Number of Reported Claims | claim	8,913
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract]
Cumulative Paid Losses and Allocated LAE, Net	$ 84.7