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 | Mar. 31, 2023 | Dec. 31, 2022 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | $ 7,825.9 | $ 7,544.4 |
Other liabilities | 80.5 | 75.3 |
Liabilities related to separate accounts | 10.2 | 10.1 |
Total financial liabilities | 90.7 | 85.4 |
Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 942.1 | 840.3 |
Other liabilities | 64.5 | 60.1 |
Liabilities related to separate accounts | 5 | 4.8 |
Total financial liabilities | 69.5 | 64.9 |
Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 6,765 | 6,607.3 |
Other liabilities | 1 | 0.2 |
Liabilities related to separate accounts | 5.2 | 5.3 |
Total financial liabilities | 6.2 | 5.5 |
Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 118.8 | 96.8 |
Other liabilities | 15 | 15 |
Liabilities related to separate accounts | 0 | 0 |
Total financial liabilities | 15 | 15 |
U.S. government and government agencies and authorities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 74.6 | 86.4 |
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 | 74.6 | 86.4 |
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 | 150.3 | 137.5 |
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 | 150.3 | 137.5 |
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 | 424.5 | 396.3 |
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 | 424.5 | 396.3 |
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 | 767.8 | 696.3 |
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 | 694.1 | 635.9 |
Asset-backed | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 73.7 | 60.4 |
Commercial mortgage-backed | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 410.1 | 402.3 |
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 | 410.1 | 402.3 |
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 | 442.9 | 438 |
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 | 442.9 | 438 |
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 | 3,011.1 | 2,961.1 |
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 | 2,975.6 | 2,932.3 |
U.S. corporate | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 35.5 | 28.8 |
Foreign corporate | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 1,276.5 | 1,165.8 |
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 | 1,267.1 | 1,158.4 |
Foreign corporate | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 9.4 | 7.4 |
Mutual funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 34.5 | 32.7 |
Mutual funds | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 34.5 | 32.7 |
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 | 22.6 | 23.9 |
Common Stock | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 21.9 | 23.2 |
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 | 0 | 0 |
Non-redeemable preferred stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 223.4 | 224.7 |
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 | 223.4 | 224.7 |
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 | 111.9 | 119.9 |
Short-term investments | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 81.3 | 72.2 |
Short-term investments | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 30.6 | 47.7 |
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 | 64.7 | 60.3 |
Other investments | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 64.5 | 60.1 |
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 | 0.2 | 0.2 |
Cash equivalents | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 800.8 | 789.1 |
Cash equivalents | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 734.9 | 647.3 |
Cash equivalents | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 65.9 | 141.8 |
Cash equivalents | 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 | 10.1 |
Assets held in separate accounts | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 5 | 4.8 |
Assets held in separate accounts | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total financial assets | 5.2 | 5.3 |
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 |