Fair Value of Financial Instruments, Fair Value Inputs (Details) - USD ($) $ in Millions | Mar. 31, 2022 | Dec. 31, 2021 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | $ 2,271 | $ 2,342.7 |
Equity Securities, FV-NI | 296.8 | 338.8 |
Short-term investments at fair value (amortized cost $3.2 at March 31, 2022 and $10.5 at December 31, 2021) | 3.2 | 10.5 |
Investments | 2,617 | 2,736 |
Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 296.8 | 338.8 |
Short-term investments at fair value (amortized cost $3.2 at March 31, 2022 and $10.5 at December 31, 2021) | 1 | 0 |
Investments | 297.8 | 338.8 |
Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 0 | 0 |
Short-term investments at fair value (amortized cost $3.2 at March 31, 2022 and $10.5 at December 31, 2021) | 2.2 | 10.5 |
Investments | 2,273.2 | 2,353.2 |
Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 0 | 0 |
Short-term investments at fair value (amortized cost $3.2 at March 31, 2022 and $10.5 at December 31, 2021) | 0 | 0 |
Investments | 0 | 0 |
Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 2,271 | 2,342.7 |
Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
US Treasury Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 63.2 | 65.7 |
US Treasury Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
US Treasury Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 63.2 | 65.7 |
US Treasury Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
US Government Agencies Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 2.3 | 2.4 |
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 2.3 | 2.4 |
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
US States and Political Subdivisions Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 372.9 | 436.1 |
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 372.9 | 436.1 |
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Corporate Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 1,045.1 | 1,080.3 |
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 1,045.1 | 1,080.3 |
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Residential Mortgage Backed Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 289.4 | 321.8 |
Residential Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Residential Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 289.4 | 321.8 |
Residential Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Commercial Mortgage Backed Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 66.5 | 92.3 |
Commercial Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Commercial Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 66.5 | 92.3 |
Commercial Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Asset-backed Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 67.5 | 68.5 |
Asset-backed Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Asset-backed Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 67.5 | 68.5 |
Asset-backed Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Foreign Government Debt | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 11.6 | 12.5 |
Foreign Government Debt | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Foreign Government Debt | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 11.6 | 12.5 |
Foreign Government Debt | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Collateralized Loan Obligations [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 183.3 | 85.4 |
Collateralized Loan Obligations [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Collateralized Loan Obligations [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 183.3 | 85.4 |
Collateralized Loan Obligations [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Other Debt Obligations [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 169.2 | 177.7 |
Other Debt Obligations [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Other Debt Obligations [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 169.2 | 177.7 |
Other Debt Obligations [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Industrial and Miscellaneous [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 248.3 | 283.1 |
Industrial and Miscellaneous [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 248.3 | 283.1 |
Industrial and Miscellaneous [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 0 | 0 |
Industrial and Miscellaneous [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 0 | 0 |
Other [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 48.5 | 55.7 |
Other [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 48.5 | 55.7 |
Other [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 0 | 0 |
Other [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | $ 0 | $ 0 |