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Property and Casualty Insurance Reserves - Short-duration Insurance Contracts, Claims Development (Details) $ in Millions | Dec. 31, 2021 USD ($) claim | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
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Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Loss and Allocated LAE Reserves, Net of Reinsurance | $ 2,491.9 |  |  |  |
Specialty Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 7,281.3 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 5,581.5 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | 22.5 |  |  |  |
Loss and Allocated LAE Reserves, Net of Reinsurance | 1,722.3 |  |  |  |
Specialty Personal Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 3,317.3 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 3,255.3 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | 1.6 |  |  |  |
Loss and Allocated LAE Reserves, Net of Reinsurance | 63.6 |  |  |  |
Commercial Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 729.5 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 443.2 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | 14.1 |  |  |  |
Loss and Allocated LAE Reserves, Net of Reinsurance | 300.4 |  |  |  |
Commercial Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 158.5 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 149.0 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | 0.4 |  |  |  |
Loss and Allocated LAE Reserves, Net of Reinsurance | 9.9 |  |  |  |
Preferred Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 853.9 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 601.9 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | 10.2 |  |  |  |
Loss and Allocated LAE Reserves, Net of Reinsurance | 262.2 |  |  |  |
Preferred Personal Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 558.0 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 552.6 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | 0.0 |  |  |  |
Loss and Allocated LAE Reserves, Net of Reinsurance | 5.4 |  |  |  |
Homeowners Insurance |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 892.0 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 813.9 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | 3.3 |  |  |  |
Loss and Allocated LAE Reserves, Net of Reinsurance | 81.4 |  |  |  |
2015 | Specialty Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 1,197.4 | $ 1,181.6 | $ 1,185.6 | $ 1,175.6 | $ 1,170.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 15.1 |  |  |  |
Cumulative Number of Reported Claims | claim | 458,261 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 1,169.3 | 1,141.9 | 1,092.9 | 945.5 | 498.3
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, | 522.4 | 522.5 | 522.0 | 522.5 | 533.1
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (0.1) |  |  |  |
Cumulative Number of Reported Claims | claim | 296,392 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 522.5 | 522.7 | 523.0 | 525.6 | 493.2
2015 | Commercial Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 114.4 | 114.3 | 118.3 | 120.0 | 120.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 2.3 |  |  |  |
Cumulative Number of Reported Claims | claim | 19,967 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 107.2 | 101.7 | 90.7 | 72.3 | 36.3
2015 | Commercial Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 23.4 | 23.4 | 23.5 | 23.5 | 24.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.0 |  |  |  |
Cumulative Number of Reported Claims | claim | 9,792 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 23.4 | 23.4 | 23.4 | 23.5 | 22.2
2015 | Preferred Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 160.0 | 158.2 | 155.8 | 157.8 | 164.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.6 |  |  |  |
Cumulative Number of Reported Claims | claim | 33,734 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 150.6 | 143.2 | 134.1 | 108.9 | 59.2
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, | 105.2 | 105.1 | 105.2 | 105.8 | 109.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.0 |  |  |  |
Cumulative Number of Reported Claims | claim | 62,685 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 105.2 | 105.1 | 105.2 | 106.1 | 104.4
2015 | Homeowners Insurance |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 243.9 | 243.8 | 245.2 | 259.5 | 261.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 1.3 |  |  |  |
Cumulative Number of Reported Claims | claim | 19,619 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 241.7 | 239.5 | 235.7 | 242.5 | $ 165.8
2016 | Specialty Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 1,314.5 | 1,307.8 | 1,310.5 | 1,324.0 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 24.0 |  |  |  |
Cumulative Number of Reported Claims | claim | 508,623 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 1,265.5 | 1,211.8 | 1,050.8 | 541.3 |
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, | 547.8 | 548.0 | 548.6 | 558.9 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (0.6) |  |  |  |
Cumulative Number of Reported Claims | claim | 309,577 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 548.3 | 549.0 | 553.1 | 509.4 |
2016 | Commercial Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 110.9 | 113.0 | 116.5 | 123.2 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 4.7 |  |  |  |
Cumulative Number of Reported Claims | claim | 20,195 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 98.1 | 88.1 | 68.8 | 36.8 |
2016 | Commercial Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 23.6 | 23.6 | 23.5 | 23.6 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.1 |  |  |  |
Cumulative Number of Reported Claims | claim | 9,569 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 23.6 | 23.6 | 23.6 | 21.7 |
2016 | Preferred Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 163.4 | 161.7 | 156.3 | 157.6 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 1.7 |  |  |  |
Cumulative Number of Reported Claims | claim | 31,922 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 147.4 | 132.7 | 107.6 | 55.5 |
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, | 110.4 | 110.4 | 111.0 | 113.9 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.1 |  |  |  |
Cumulative Number of Reported Claims | claim | 61,921 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 110.3 | 110.4 | 111.4 | 107.2 |
2016 | Homeowners Insurance |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 185.3 | 183.6 | 183.0 | 185.9 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 4.1 |  |  |  |
Cumulative Number of Reported Claims | claim | 16,212 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 183.1 | 180.0 | 180.2 | $ 127.4 |
2017 | Specialty Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 1,506.1 | 1,494.7 | 1,461.5 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 54.8 |  |  |  |
Cumulative Number of Reported Claims | claim | 546,521 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 1,382.0 | 1,200.7 | 567.3 |  |
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, | 629.6 | 630.3 | 624.3 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (10.5) |  |  |  |
Cumulative Number of Reported Claims | claim | 324,321 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 630.6 | 634.8 | 570.8 |  |
2017 | Commercial Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 126.6 | 126.1 | 128.4 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 11.5 |  |  |  |
Cumulative Number of Reported Claims | claim | 19,572 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 99.5 | 75.7 | 32.4 |  |
2017 | Commercial Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 26.9 | 27.1 | 26.0 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (0.5) |  |  |  |
Cumulative Number of Reported Claims | claim | 9,305 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 26.8 | 26.9 | 23.0 |  |
2017 | Preferred Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 200.0 | 195.5 | 172.2 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 4.9 |  |  |  |
Cumulative Number of Reported Claims | claim | 34,633 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 160.8 | 127.9 | 62.7 |  |
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, | 125.9 | 125.8 | 126.4 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.3 |  |  |  |
Cumulative Number of Reported Claims | claim | 67,181 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 125.6 | 126.5 | 120.7 |  |
2017 | Homeowners Insurance |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 163.1 | 161.8 | 162.9 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 2.5 |  |  |  |
Cumulative Number of Reported Claims | claim | 14,670 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 157.7 | 150.4 | $ 111.1 |  |
2018 | Specialty Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 1,406.4 | 1,401.2 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 111.6 |  |  |  |
Cumulative Number of Reported Claims | claim | 474,346 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 1,107.6 | 555.2 |  |  |
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, | 659.5 | 650.5 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (6.4) |  |  |  |
Cumulative Number of Reported Claims | claim | 296,167 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 663.8 | 585.5 |  |  |
2018 | Commercial Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 152.0 | 140.5 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 25.5 |  |  |  |
Cumulative Number of Reported Claims | claim | 19,354 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 87.6 | 37.0 |  |  |
2018 | Commercial Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 32.2 | 31.9 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.0 |  |  |  |
Cumulative Number of Reported Claims | claim | 11,011 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 31.9 | 26.2 |  |  |
2018 | Preferred Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 153.6 | 148.9 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 16.1 |  |  |  |
Cumulative Number of Reported Claims | claim | 24,438 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 92.8 | 44.4 |  |  |
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, | 98.0 | 96.1 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (0.5) |  |  |  |
Cumulative Number of Reported Claims | claim | 47,603 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 98.4 | 90.9 |  |  |
2018 | Homeowners Insurance |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 149.8 | 157.0 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 4.3 |  |  |  |
Cumulative Number of Reported Claims | claim | 13,887 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 130.8 | $ 94.6 |  |  |
2019 | Specialty Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 1,856.9 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 607.1 |  |  |  |
Cumulative Number of Reported Claims | claim | 544,207 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 657.1 |  |  |  |
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, | 958.0 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 2.1 |  |  |  |
Cumulative Number of Reported Claims | claim | 348,244 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 890.1 |  |  |  |
2019 | Commercial Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 225.6 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 99.1 |  |  |  |
Cumulative Number of Reported Claims | claim | 24,690 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 50.8 |  |  |  |
2019 | Commercial Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 52.4 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.4 |  |  |  |
Cumulative Number of Reported Claims | claim | 16,494 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 43.3 |  |  |  |
2019 | Preferred Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 176.9 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 54.1 |  |  |  |
Cumulative Number of Reported Claims | claim | 25,149 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 50.3 |  |  |  |
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, | 118.5 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (0.7) |  |  |  |
Cumulative Number of Reported Claims | claim | 50,533 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 113.1 |  |  |  |
2019 | Homeowners Insurance |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 149.9 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 19.1 |  |  |  |
Cumulative Number of Reported Claims | claim | 12,422 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 100.6 |  |  |  |

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