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Property and Casualty Insurance Reserves - Short-duration Insurance Contracts, Claims Development (Details) $ in Millions | Dec. 31, 2020 USD ($) claim | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
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Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Property and Casualty Insurance Reserves, Net of Reinsurance | $ 1,756.7 |  |  |  |
Specialty Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 5,668.0 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 4,559.4 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | 3.9 |  |  |  |
Property and Casualty Insurance Reserves, Net of Reinsurance | 1,112.5 |  |  |  |
Specialty Personal Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 2,600.6 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 2,548.7 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | 1.6 |  |  |  |
Property and Casualty Insurance Reserves, Net of Reinsurance | 53.5 |  |  |  |
Commercial Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 609.5 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 412.2 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | 11.8 |  |  |  |
Property and Casualty Insurance Reserves, Net of Reinsurance | 209.1 |  |  |  |
Commercial Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 130.2 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 124.3 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | 0.4 |  |  |  |
Property and Casualty Insurance Reserves, Net of Reinsurance | 6.3 |  |  |  |
Preferred Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 841.5 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 617.5 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | 8.6 |  |  |  |
Property and Casualty Insurance Reserves, Net of Reinsurance | 232.6 |  |  |  |
Preferred Personal Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 543.6 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 539.1 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | (0.1) |  |  |  |
Property and Casualty Insurance Reserves, Net of Reinsurance | 4.4 |  |  |  |
Homeowners Insurance |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 947.3 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | 864.0 |  |  |  |
Outstanding Loss and Allocated LAE Reserves on Accident Years before 2015, Net of Reinsurance | 5.1 |  |  |  |
Property and Casualty Insurance Reserves, Net of Reinsurance | 88.4 |  |  |  |
2015 | Specialty Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 1,022.7 | $ 1,026.0 | $ 1,027.2 | $ 1,021.6 | $ 969.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 4.7 |  |  |  |
Cumulative Number of Reported Claims | claim | 417,219 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 1,008.3 | 987.7 | 943.4 | 831.1 | 459.7
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, | 457.4 | 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 | 246,239 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 457.5 | 457.5 | 458.0 | 460.2 | 436.4
2015 | Commercial Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 115.6 | 117.7 | 115.6 | 112.4 | 120.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 1.1 |  |  |  |
Cumulative Number of Reported Claims | claim | 20,427 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 109.7 | 102.3 | 89.7 | 71.6 | 36.2
2015 | 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 | 24.2 | 24.1 | 24.2 | 24.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.0 |  |  |  |
Cumulative Number of Reported Claims | claim | 10,561 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 24.2 | 24.2 | 24.1 | 24.2 | 22.4
2015 | Preferred Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 177.2 | 176.8 | 179.1 | 174.5 | 162.1
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 1.6 |  |  |  |
Cumulative Number of Reported Claims | claim | 36,766 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 169.3 | 161.1 | 145.6 | 114.6 | 61.2
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, | 106.2 | 106.2 | 106.3 | 106.6 | 106.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.0 |  |  |  |
Cumulative Number of Reported Claims | claim | 65,191 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 106.2 | 106.3 | 106.3 | 106.9 | 105.2
2015 | Homeowners Insurance |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 201.1 | 202.2 | 204.2 | 201.7 | 200.3
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.3 |  |  |  |
Cumulative Number of Reported Claims | claim | 20,019 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 199.5 | 198.9 | 195.8 | 190.1 | $ 141.2
2016 | Specialty Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 999.1 | 1,004.5 | 999.9 | 997.7 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 13.7 |  |  |  |
Cumulative Number of Reported Claims | claim | 397,059 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 967.7 | 926.7 | 808.6 | 441.9 |
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, | 465.6 | 465.1 | 465.6 | 475.6 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.1 |  |  |  |
Cumulative Number of Reported Claims | claim | 251,935 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 465.9 | 466.0 | 468.7 | 443.0 |
2016 | Commercial Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 114.3 | 118.3 | 120.0 | 120.5 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 2.9 |  |  |  |
Cumulative Number of Reported Claims | claim | 19,951 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 101.7 | 90.7 | 72.3 | 36.3 |
2016 | Commercial Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 23.4 | 23.5 | 23.5 | 24.2 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.1 |  |  |  |
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 | 23.5 | 22.2 |
2016 | Preferred Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 158.2 | 155.8 | 157.8 | 164.4 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 1.9 |  |  |  |
Cumulative Number of Reported Claims | claim | 33,731 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 143.2 | 134.1 | 108.9 | 59.2 |
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, | 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 | 62,385 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 105.1 | 105.2 | 106.1 | 104.4 |
2016 | Homeowners Insurance |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 243.8 | 245.2 | 259.5 | 261.2 |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.6 |  |  |  |
Cumulative Number of Reported Claims | claim | 20,796 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 239.5 | 235.7 | 242.5 | $ 165.8 |
2017 | Specialty Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 1,120.0 | 1,119.1 | 1,128.1 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 33.9 |  |  |  |
Cumulative Number of Reported Claims | claim | 447,610 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 1,039.3 | 903.8 | 467.5 |  |
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, | 496.4 | 496.9 | 504.9 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (0.4) |  |  |  |
Cumulative Number of Reported Claims | claim | 270,000 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 497.4 | 501.5 | 463.6 |  |
2017 | Commercial Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 113.0 | 116.5 | 123.2 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 12.6 |  |  |  |
Cumulative Number of Reported Claims | claim | 20,146 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 88.1 | 68.8 | 36.8 |  |
2017 | Commercial Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 23.6 | 23.5 | 23.6 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 0.1 |  |  |  |
Cumulative Number of Reported Claims | claim | 9,567 |  |  |  |
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 | 21.7 |  |
2017 | Preferred Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 161.7 | 156.3 | 157.6 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 6.1 |  |  |  |
Cumulative Number of Reported Claims | claim | 32,246 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 132.7 | 107.6 | 55.5 |  |
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, | 110.4 | 111.0 | 113.9 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (0.1) |  |  |  |
Cumulative Number of Reported Claims | claim | 60,707 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 110.4 | 111.4 | 107.2 |  |
2017 | Homeowners Insurance |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 183.6 | 183.0 | 185.9 |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 3.0 |  |  |  |
Cumulative Number of Reported Claims | claim | 17,104 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 180.0 | 180.2 | $ 127.4 |  |
2018 | Specialty Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 1,306.9 | 1,270.7 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 91.4 |  |  |  |
Cumulative Number of Reported Claims | claim | 485,455 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 1,052.5 | 497.2 |  |  |
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, | 581.0 | 574.7 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (10.7) |  |  |  |
Cumulative Number of Reported Claims | claim | 286,954 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 585.7 | 525.8 |  |  |
2018 | Commercial Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 126.1 | 128.4 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 19.4 |  |  |  |
Cumulative Number of Reported Claims | claim | 19,404 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 75.7 | 32.4 |  |  |
2018 | Commercial Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 27.1 | 26.0 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (0.3) |  |  |  |
Cumulative Number of Reported Claims | claim | 9,290 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 26.9 | 23.0 |  |  |
2018 | Preferred Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 195.5 | 172.2 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 20.0 |  |  |  |
Cumulative Number of Reported Claims | claim | 35,891 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 127.9 | 62.7 |  |  |
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, | 125.8 | 126.4 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (0.7) |  |  |  |
Cumulative Number of Reported Claims | claim | 63,776 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 126.5 | 120.7 |  |  |
2018 | Homeowners Insurance |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 161.8 | 162.9 |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 4.1 |  |  |  |
Cumulative Number of Reported Claims | claim | 15,311 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 150.4 | $ 111.1 |  |  |
2019 | Specialty Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 1,219.3 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 368.6 |  |  |  |
Cumulative Number of Reported Claims | claim | 399,216 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 491.6 |  |  |  |
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, | 600.2 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 52.0 |  |  |  |
Cumulative Number of Reported Claims | claim | 252,237 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 542.2 |  |  |  |
2019 | Commercial Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 140.5 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 60.5 |  |  |  |
Cumulative Number of Reported Claims | claim | 16,457 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 37.0 |  |  |  |
2019 | Commercial Automobile Insurance—Physical Damage |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 31.9 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 4.6 |  |  |  |
Cumulative Number of Reported Claims | claim | 10,936 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 26.2 |  |  |  |
2019 | Preferred Personal Automobile Insurance—Liability |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 148.9 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 45.8 |  |  |  |
Cumulative Number of Reported Claims | claim | 23,543 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 44.4 |  |  |  |
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, | 96.1 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ (0.7) |  |  |  |
Cumulative Number of Reported Claims | claim | 43,539 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 90.9 |  |  |  |
2019 | Homeowners Insurance |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |
Incurred Losses and Allocated LAE, Net of Reinsurance For the Years Ended December 31, | 157.0 |  |  |  |
Total of IBNR Liabilities Plus Expected Development on Reported Claims | $ 17.5 |  |  |  |
Cumulative Number of Reported Claims | claim | 12,529 |  |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net [Abstract] |  |  |  |  |
Cumulative Paid Losses and Allocated LAE, Net | $ 94.6 |  |  |  |

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