Fair Value Disclosures - Fair Value for Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions | Jun. 30, 2022 | Dec. 31, 2021 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | $ 7,291.6 | $ 9,198.2 |
Other liabilities | 69.4 | 76.4 |
Liabilities related to separate accounts | 10.2 | 11.8 |
Total financial liabilities | 79.6 | 88.2 |
Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 712.1 | 1,519.9 |
Other liabilities | 65.7 | 72.4 |
Liabilities related to separate accounts | 6.7 | 7.7 |
Total financial liabilities | 72.4 | 80.1 |
Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 6,571.4 | 7,536.2 |
Other liabilities | 0 | 0 |
Liabilities related to separate accounts | 3.5 | 4.1 |
Total financial liabilities | 3.5 | 4.1 |
Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 8.1 | 142.1 |
Other liabilities | 3.7 | 4 |
Liabilities related to separate accounts | 0 | 0 |
Total financial liabilities | 3.7 | 4 |
U.S. government and government agencies and authorities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 82.5 | 85 |
U.S. government and government agencies and authorities | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
U.S. government and government agencies and authorities | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 82.5 | 85 |
U.S. government and government agencies and authorities | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
States, municipalities and political subdivisions | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 164.7 | 148.5 |
States, municipalities and political subdivisions | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
States, municipalities and political subdivisions | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 164.7 | 148.5 |
States, municipalities and political subdivisions | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
Foreign governments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 388.6 | 437.7 |
Foreign governments | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
Foreign governments | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 388.6 | 437.7 |
Foreign governments | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
Asset-backed | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 562.1 | 423 |
Asset-backed | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
Asset-backed | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 562.1 | 423 |
Asset-backed | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
Commercial mortgage-backed | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 417.5 | 473.7 |
Commercial mortgage-backed | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
Commercial mortgage-backed | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 417.5 | 473.7 |
Commercial mortgage-backed | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
Residential mortgage-backed | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 518 | 601.9 |
Residential mortgage-backed | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
Residential mortgage-backed | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 518 | 601.9 |
Residential mortgage-backed | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
U.S. corporate | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 2,993.2 | 3,803.1 |
U.S. corporate | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
U.S. corporate | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 2,990.1 | 3,799.7 |
U.S. corporate | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 3.1 | 3.4 |
Foreign corporate | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 1,182.2 | 1,242.4 |
Foreign corporate | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
Foreign corporate | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 1,179 | 1,238.8 |
Foreign corporate | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 3.2 | 3.6 |
Mutual funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 34.5 | 33.3 |
Mutual funds | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 34.5 | 33.3 |
Mutual funds | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
Mutual funds | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
Common Stock | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 40.3 | 151.1 |
Common Stock | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 38.4 | 15.5 |
Common Stock | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0.7 | 0.7 |
Common Stock | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 1.2 | 134.9 |
Non-redeemable preferred stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 236.2 | 261.3 |
Non-redeemable preferred stocks | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
Non-redeemable preferred stocks | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 236.2 | 261.3 |
Non-redeemable preferred stocks | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
Short-term investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 139.7 | 207.2 |
Short-term investments | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 132.3 | 200.1 |
Short-term investments | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 7.4 | 7.1 |
Short-term investments | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
Other investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 66.3 | 72.6 |
Other investments | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 65.7 | 72.4 |
Other investments | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
Other investments | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0.6 | 0.2 |
Cash equivalents | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 449.2 | 1,243.9 |
Cash equivalents | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 434.5 | 1,190.9 |
Cash equivalents | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 14.7 | 53 |
Cash equivalents | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
Other assets | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 6.4 | 1.7 |
Other assets | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
Other assets | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 6.4 | 1.7 |
Other assets | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 0 | 0 |
Assets held in separate accounts | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 10.2 | 11.8 |
Assets held in separate accounts | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 6.7 | 7.7 |
Assets held in separate accounts | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 3.5 | 4.1 |
Assets held in separate accounts | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | $ 0 | $ 0 |