Fair Value Measurements - Fair Value Inputs (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	$ 7,835.0	$ 7,605.9
Equity securities	957.7	858.5
Investments in Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value		7,605.9
U.S. Government and Government Agencies and Authorities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	522.3	585.3
States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	1,805.8	1,589.5
Foreign Governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	5.1	5.2
Bonds and Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	4,475.4	4,425.4
Redeemable Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	7.5	7.5
Collateralized Loan Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	770.2	767.7
Other Mortgage- and Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	248.7	225.3
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible debt at fair value	43.6	39.9
Total	8,836.3	8,504.3
Fair Value, Measurements, Recurring | Investments in Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	7,835.0	7,605.9
Fair Value, Measurements, Recurring | U.S. Government and Government Agencies and Authorities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	522.3	585.3
Fair Value, Measurements, Recurring | States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	1,805.8	1,589.5
Fair Value, Measurements, Recurring | Foreign Governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	5.1	5.2
Fair Value, Measurements, Recurring | Bonds and Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	4,475.4	4,425.4
Fair Value, Measurements, Recurring | Redeemable Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	7.5
Fair Value, Measurements, Recurring | Collateralized Loan Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	770.2	767.7
Fair Value, Measurements, Recurring | Other Mortgage- and Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	248.7	225.3
Fair Value, Measurements, Recurring | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	957.7	858.5
Fair Value, Measurements, Recurring | Exchange Traded Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	564.3	496.4
Fair Value, Measurements, Recurring | Limited Liability Companies and Limited Partnerships
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	316.4	292.2
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible debt at fair value	0.0	0.0
Total	725.0	639.5
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Investments in Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	137.9	134.0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government and Government Agencies and Authorities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	137.9	134.0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	0.0	0.0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign Governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	0.0	0.0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Bonds and Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	0.0	0.0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Redeemable Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	0.0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Redeemable Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		0.0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Collateralized Loan Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	0.0	0.0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Mortgage- and Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	0.0	0.0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	587.1	505.5
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Exchange Traded Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	564.3	496.4
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Limited Liability Companies and Limited Partnerships
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.0	0.0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible debt at fair value	43.6	39.9
Total	7,330.1	7,123.4
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Investments in Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	7,235.7	7,022.7
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Government and Government Agencies and Authorities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	384.4	451.3
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	1,805.8	1,589.5
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Foreign Governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	5.1	5.2
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Bonds and Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	4,066.2	3,992.4
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Redeemable Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	1.3
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Redeemable Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		1.3
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Collateralized Loan Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	742.4	767.7
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Other Mortgage- and Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	230.5	215.3
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	50.8	60.8
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Exchange Traded Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.0	0.0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Limited Liability Companies and Limited Partnerships
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.0	0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible debt at fair value	0.0	0.0
Total	464.8	449.2
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Investments in Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	461.4	449.2
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | U.S. Government and Government Agencies and Authorities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	0.0	0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	0.0	0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Foreign Governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	0.0	0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Bonds and Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	409.2	433.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Redeemable Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	6.2
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Redeemable Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		6.2
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Collateralized Loan Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	27.8	0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Other Mortgage- and Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	18.2	10.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	3.4	0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Exchange Traded Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.0	0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Limited Liability Companies and Limited Partnerships
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.0	0.0
Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	34.8	43.7
Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	21.7	10.4
Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.0	0.0
Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	20.0	8.7
Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	34.8	43.7
Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.0	1.7
Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.0	0.0
Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	1.7	0.0
Other Industries | Fair Value, Measurements, Recurring | Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	17.6	15.4
Other Industries | Fair Value, Measurements, Recurring | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	2.9	0.4
Other Industries | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.0	0.0
Other Industries | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	2.8	0.4
Other Industries | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	15.9	15.4
Other Industries | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.1	0.0
Other Industries | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	1.7	0.0
Other Industries | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities		0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible debt at fair value	0.0	0.0
Total	316.4	292.2
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Investments in Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	0.0	0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring | U.S. Government and Government Agencies and Authorities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	0.0	0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring | States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	0.0	0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Foreign Governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	0.0	0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Bonds and Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	0.0	0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Redeemable Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Redeemable Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value		7.5
Available-for-sale Securities		0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Collateralized Loan Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	0.0	0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Other Mortgage- and Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities at fair value	0.0	0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	316.4	292.2
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Exchange Traded Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.0	0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Limited Liability Companies and Limited Partnerships
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	316.4	292.2
Measured at Net Asset Value | Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.0	0.0
Measured at Net Asset Value | Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.0	0.0
Measured at Net Asset Value | Other Industries | Fair Value, Measurements, Recurring | Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.0	0.0
Measured at Net Asset Value | Other Industries | Fair Value, Measurements, Recurring | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	$ 0.0	$ 0.0