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Property and Casualty Insurance Reserves - Short-duration Insurance Contracts, Claims Development (Details) $ in Millions | Dec. 31, 2019 USD ($) claim | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 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 | $ 1,725.1 |  |  |  |
Specialty Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 5,389.9 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 4,269.4 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | 20.4 |  |  |  |
Loss and Allocated LAE Reserves, Net of Reinsurance | 1,140.9 |  |  |  |
Specialty Personal Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 2,400.3 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 2,356.9 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | (1.2) |  |  |  |
Loss and Allocated LAE Reserves, Net of Reinsurance | 42.2 |  |  |  |
Commercial Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 591.6 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 401.8 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | 1.4 |  |  |  |
Loss and Allocated LAE Reserves, Net of Reinsurance | 191.2 |  |  |  |
Commercial Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 118.9 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 115.9 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | 0.0 |  |  |  |
Loss and Allocated LAE Reserves, Net of Reinsurance | 3.0 |  |  |  |
Preferred Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 838.2 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 634.8 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | 9.5 |  |  |  |
Loss and Allocated LAE Reserves, Net of Reinsurance | 212.9 |  |  |  |
Preferred Personal Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 549.4 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 544.2 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | (0.1) |  |  |  |
Loss and Allocated LAE Reserves, Net of Reinsurance | 5.1 |  |  |  |
Homeowners Insurance |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 957.7 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 888.6 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | 3.2 |  |  |  |
Loss and Allocated LAE Reserves, Net of Reinsurance | 72.3 |  |  |  |
2015 | Specialty Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 969.6 | $ 972.3 | $ 965.1 | $ 855.1 | $ 965.1
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 8.1 |  |  |  |
Cumulative Number of Reported Claims | claim | 401,025 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 954.0 | 934.7 | 898.9 | 794.9 | 450.5
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, | 406.6 | 406.5 | 406.7 | 407.1 | 403.8
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.9 |  |  |  |
Cumulative Number of Reported Claims | claim | 231,007 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 406.1 | 406.4 | 406.7 | 408.8 | 382.4
2015 | Commercial Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 110.7 | 112.3 | 112.9 | 106.9 | 102.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.6 |  |  |  |
Cumulative Number of Reported Claims | claim | 19,292 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 107.6 | 100.3 | 88.6 | 70.2 | 34.8
2015 | Commercial Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 21.7 | 21.7 | 21.7 | 21.6 | 21.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.0 |  |  |  |
Cumulative Number of Reported Claims | claim | 9,643 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 21.7 | 21.7 | 21.7 | 21.6 | 20.2
2015 | Preferred Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 177.1 | 177.6 | 176.5 | 171.8 | 168.3
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.9 |  |  |  |
Cumulative Number of Reported Claims | claim | 38,961 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 169.3 | 163.1 | 147.5 | 122.4 | 73.1
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, | 100.6 | 100.6 | 100.6 | 100.7 | 101.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.0 |  |  |  |
Cumulative Number of Reported Claims | claim | 68,409 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 100.6 | 100.6 | 100.7 | 101.0 | 100.1
2015 | Homeowners Insurance |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 164.4 | 164.3 | 163.2 | 164.9 | 178.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.5 |  |  |  |
Cumulative Number of Reported Claims | claim | 19,731 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 162.7 | 161.3 | 158.0 | 154.4 | $ 116.9
2016 | Specialty Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 1,026.0 | 1,027.2 | 1,021.6 | 969.4 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 22.5 |  |  |  |
Cumulative Number of Reported Claims | claim | 417,016 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 987.7 | 943.4 | 831.1 | 459.7 |
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, | 457.0 | 456.9 | 456.9 | 462.2 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (0.1) |  |  |  |
Cumulative Number of Reported Claims | claim | 246,183 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 457.5 | 458.0 | 460.2 | 436.4 |
2016 | Commercial Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 117.7 | 115.6 | 112.4 | 120.5 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 3.5 |  |  |  |
Cumulative Number of Reported Claims | claim | 20,417 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 102.3 | 89.7 | 71.6 | 36.2 |
2016 | Commercial Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 24.2 | 24.1 | 24.2 | 24.2 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.0 |  |  |  |
Cumulative Number of Reported Claims | claim | 10,560 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 24.2 | 24.1 | 24.2 | 22.4 |
2016 | Preferred Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 176.8 | 179.1 | 174.5 | 162.1 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 2.1 |  |  |  |
Cumulative Number of Reported Claims | claim | 36,721 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 161.1 | 145.6 | 114.6 | 61.2 |
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, | 106.2 | 106.3 | 106.6 | 106.6 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (0.1) |  |  |  |
Cumulative Number of Reported Claims | claim | 65,348 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 106.3 | 106.3 | 106.9 | 105.2 |
2016 | Homeowners Insurance |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 202.2 | 204.2 | 201.7 | 200.3 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 1.4 |  |  |  |
Cumulative Number of Reported Claims | claim | 20,380 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 198.9 | 195.8 | 190.1 | $ 141.2 |
2017 | Specialty Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 1,004.5 | 999.9 | 997.7 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 40.6 |  |  |  |
Cumulative Number of Reported Claims | claim | 396,578 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 926.7 | 808.6 | 441.9 |  |
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, | 465.1 | 465.6 | 475.6 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.0 |  |  |  |
Cumulative Number of Reported Claims | claim | 251,872 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 466.0 | 468.7 | 443.0 |  |
2017 | Commercial Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 118.3 | 120.0 | 120.5 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 9.1 |  |  |  |
Cumulative Number of Reported Claims | claim | 19,925 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 90.7 | 72.3 | 36.3 |  |
2017 | Commercial Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 23.5 | 23.5 | 24.2 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.2 |  |  |  |
Cumulative Number of Reported Claims | claim | 9,788 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 23.4 | 23.5 | 22.2 |  |
2017 | Preferred Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 155.8 | 157.8 | 164.4 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 4.5 |  |  |  |
Cumulative Number of Reported Claims | claim | 33,663 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 134.1 | 108.9 | 59.2 |  |
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, | 105.2 | 105.8 | 109.2 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (0.1) |  |  |  |
Cumulative Number of Reported Claims | claim | 62,553 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 105.2 | 106.1 | 104.4 |  |
2017 | Homeowners Insurance |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 245.2 | 259.5 | 261.2 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.9 |  |  |  |
Cumulative Number of Reported Claims | claim | 20,982 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 235.7 | 242.5 | $ 165.8 |  |
2018 | Specialty Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 1,119.1 | 1,128.1 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 93.0 |  |  |  |
Cumulative Number of Reported Claims | claim | 445,229 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 903.8 | 467.5 |  |  |
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, | 496.9 | 504.9 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (1.6) |  |  |  |
Cumulative Number of Reported Claims | claim | 269,782 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 501.5 | 463.6 |  |  |
2018 | Commercial Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 116.5 | 123.2 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 19.8 |  |  |  |
Cumulative Number of Reported Claims | claim | 19,969 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 68.8 | 36.8 |  |  |
2018 | Commercial Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 23.5 | 23.6 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.3 |  |  |  |
Cumulative Number of Reported Claims | claim | 9,558 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 23.6 | 21.7 |  |  |
2018 | Preferred Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 156.3 | 157.6 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 15.6 |  |  |  |
Cumulative Number of Reported Claims | claim | 32,020 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 107.6 | 55.5 |  |  |
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, | 111.0 | 113.9 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (0.5) |  |  |  |
Cumulative Number of Reported Claims | claim | 60,749 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 111.4 | 107.2 |  |  |
2018 | Homeowners Insurance |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 183.0 | 185.9 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 1.2 |  |  |  |
Cumulative Number of Reported Claims | claim | 17,122 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 180.2 | $ 127.4 |  |  |
2019 | Specialty Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 1,270.7 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 383.0 |  |  |  |
Cumulative Number of Reported Claims | claim | 447,558 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 497.2 |  |  |  |
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, | 574.7 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 47.5 |  |  |  |
Cumulative Number of Reported Claims | claim | 276,797 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 525.8 |  |  |  |
2019 | Commercial Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 128.4 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 59.2 |  |  |  |
Cumulative Number of Reported Claims | claim | 17,772 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 32.4 |  |  |  |
2019 | Commercial Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 26.0 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 3.5 |  |  |  |
Cumulative Number of Reported Claims | claim | 8,803 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 23.0 |  |  |  |
2019 | Preferred Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 172.2 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 51.3 |  |  |  |
Cumulative Number of Reported Claims | claim | 32,848 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 62.7 |  |  |  |
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, | 126.4 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (0.9) |  |  |  |
Cumulative Number of Reported Claims | claim | 60,548 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 120.7 |  |  |  |
2019 | Homeowners Insurance |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 162.9 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 19.2 |  |  |  |
Cumulative Number of Reported Claims | claim | 13,931 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 111.1 |  |  |  |

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