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 | Sep. 30, 2019 | Dec. 31, 2018 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | $ 16,063.6 | $ 14,307.8 |
Other liabilities | 147.6 | 104.8 |
Liabilities related to separate accounts | 1,715.2 | 1,575.7 |
Total financial liabilities | 1,862.8 | 1,680.5 |
Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 2,834.8 | 2,235.6 |
Other liabilities | 67 | 62.9 |
Liabilities related to separate accounts | 1,548.4 | 1,400.1 |
Total financial liabilities | 1,615.4 | 1,463 |
Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 13,155.1 | 11,960.8 |
Other liabilities | 0 | 0.7 |
Liabilities related to separate accounts | 166.8 | 175.6 |
Total financial liabilities | 166.8 | 176.3 |
Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 73.7 | 111.4 |
Other liabilities | 80.6 | 41.2 |
Liabilities related to separate accounts | 0 | 0 |
Total financial liabilities | 80.6 | 41.2 |
U.S. government and government agencies and authorities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 194 | 384.6 |
U.S. government and government agencies and authorities | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
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] | | |
Financial assets | 194 | 384.6 |
U.S. government and government agencies and authorities | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0 | 0 |
States, municipalities and political subdivisions | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 264.4 | 256.2 |
States, municipalities and political subdivisions | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0 | 0 |
States, municipalities and political subdivisions | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 264.4 | 256.2 |
States, municipalities and political subdivisions | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0 | 0 |
Foreign governments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 982.2 | 912.1 |
Foreign governments | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0.2 | 0.5 |
Foreign governments | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 982 | 911.6 |
Foreign governments | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0 | 0 |
Asset-backed | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 482.3 | 504.5 |
Asset-backed | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0 | 0 |
Asset-backed | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 480.7 | 504.5 |
Asset-backed | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 1.6 | 0 |
Commercial mortgage-backed | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 240 | 79.7 |
Commercial mortgage-backed | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0 | 0 |
Commercial mortgage-backed | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 214.9 | 40.8 |
Commercial mortgage-backed | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 25.1 | 38.9 |
Residential mortgage-backed | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 1,296.5 | 1,405.8 |
Residential mortgage-backed | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0 | 0 |
Residential mortgage-backed | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 1,296.5 | 1,405.8 |
Residential mortgage-backed | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0 | 0 |
U.S. corporate | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 6,585.1 | 5,593 |
U.S. corporate | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0 | 0 |
U.S. corporate | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 6,579.1 | 5,580.3 |
U.S. corporate | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 6 | 12.7 |
Foreign corporate | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 2,376.3 | 2,121.2 |
Foreign corporate | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0 | 0 |
Foreign corporate | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 2,340.2 | 2,071.7 |
Foreign corporate | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 36.1 | 49.5 |
Mutual funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 40.7 | 45 |
Mutual funds | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 40.7 | 45 |
Mutual funds | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0 | 0 |
Mutual funds | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0 | 0 |
Common Stock | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 19.5 | 15.3 |
Common Stock | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 18.8 | 14.6 |
Common Stock | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0.7 | 0.7 |
Common Stock | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0 | 0 |
Non-redeemable preferred stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 346.7 | 318.5 |
Non-redeemable preferred stocks | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0 | 0 |
Non-redeemable preferred stocks | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 344.5 | 316.3 |
Non-redeemable preferred stocks | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 2.2 | 2.2 |
Short-term investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 231 | 336 |
Short-term investments | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 129.2 | 188.9 |
Short-term investments | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 101.8 | 147.1 |
Short-term investments | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0 | 0 |
Other investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 232.6 | 224.9 |
Other investments | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 67 | 62.9 |
Other investments | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 165.6 | 161.5 |
Other investments | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0 | 0.5 |
Cash equivalents | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 1,051.2 | 527.7 |
Cash equivalents | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 1,030.5 | 523.6 |
Cash equivalents | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 20.7 | 4.1 |
Cash equivalents | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0 | 0 |
Other receivables | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | | 5 |
Other receivables | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | | 0 |
Other receivables | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | | 0 |
Other receivables | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | | 5 |
Other assets | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 5.9 | 2.6 |
Other assets | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 0 | 0 |
Other assets | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 3.2 | 0 |
Other assets | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 2.7 | 2.6 |
Assets held in separate accounts | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 1,715.2 | 1,575.7 |
Assets held in separate accounts | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 1,548.4 | 1,400.1 |
Assets held in separate accounts | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | 166.8 | 175.6 |
Assets held in separate accounts | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Financial assets | $ 0 | $ 0 |