Investments Fair Value (Details) - USD ($) $ in Millions | Mar. 31, 2022 | Dec. 31, 2021 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | $ 4,972.4 | $ 5,302.8 |
Available for sale, Fixed income securities | 10,736.2 | 10,675.7 |
Short-term investments | 739.8 | 565.7 |
Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments | 739.8 | 565.7 |
Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments | 0 | 0 |
Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments | 0 | 0 |
U.S. & Canadian Governments [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale, Fixed income securities | 2,206 | 2,158.5 |
U.S. & Canadian Governments [Member] | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale, Fixed income securities | 1,530.9 | 1,453.8 |
U.S. & Canadian Governments [Member] | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale, Fixed income securities | 675 | 704.6 |
U.S. & Canadian Governments [Member] | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale, Fixed income securities | 0 | 0 |
Tax-exempt [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale, Fixed income securities | 930.9 | 989.2 |
Tax-exempt [Member] | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale, Fixed income securities | 0 | 0 |
Tax-exempt [Member] | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale, Fixed income securities | 930.9 | 989.2 |
Tax-exempt [Member] | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale, Fixed income securities | 0 | 0 |
Corporate [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale, Fixed income securities | 7,599.2 | 7,527.9 |
Corporate [Member] | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale, Fixed income securities | 0 | 0 |
Corporate [Member] | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale, Fixed income securities | 7,588.7 | 7,517.4 |
Corporate [Member] | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale, Fixed income securities | 10.5 | 10.5 |
Equity Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 4,972.4 | 5,302.8 |
Equity Securities [Member] | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 4,970.5 | 5,300.8 |
Equity Securities [Member] | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0 | 0 |
Equity Securities [Member] | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | $ 1.8 | $ 1.9 |