Fair Value Measurements - Valuation of Assets Measured at Fair Value (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	$ 6,922,100,000	$ 6,424,200,000
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	907,300,000	684,400,000
Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible securities at fair value	37,300,000	31,500,000
Total	7,866,700,000	7,140,100,000
Fair Value, 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 securities at fair value	0	0
Total	745,000,000.0	594,200,000
Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible securities at fair value	37,300,000	31,500,000
Total	5,829,400,000	5,456,500,000
Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible securities at fair value	0	0
Total	1,044,200,000	897,400,000
Investments in Fixed Maturities | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	6,922,100,000	6,424,200,000
Investments in Fixed Maturities | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	144,300,000	156,500,000
Investments in Fixed Maturities | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	5,733,600,000	5,370,300,000
Investments in Fixed Maturities | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	1,044,200,000	897,400,000
United States Government and Government Agencies and Authorities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	815,900,000	865,700,000
United States Government and Government Agencies and Authorities | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	815,900,000	865,700,000
United States Government and Government Agencies and Authorities | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	144,300,000	156,500,000
United States Government and Government Agencies and Authorities | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	671,600,000	709,200,000
United States Government and Government Agencies and Authorities | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0	0
States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	1,515,800,000	1,619,100,000
States and Political Subdivisions | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	1,515,800,000	1,619,100,000
States and Political Subdivisions | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0	0
States and Political Subdivisions | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	1,515,800,000	1,619,100,000
States and Political Subdivisions | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0	0
Foreign Governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	16,800,000	5,900,000
Foreign Governments | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	16,800,000	5,900,000
Foreign Governments | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0	0
Foreign Governments | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	16,800,000	5,900,000
Foreign Governments | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0	0
Corporate Bonds and Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	3,859,700,000	3,393,800,000
Corporate Bonds and Notes | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	3,859,700,000	3,393,800,000
Corporate Bonds and Notes | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0	0
Corporate Bonds and Notes | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	3,450,600,000	3,011,200,000
Corporate Bonds and Notes | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	409,100,000	382,600,000
Redeemable Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	6,700,000
Redeemable Preferred Stocks | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	6,700,000
Redeemable Preferred Stocks | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0
Redeemable Preferred Stocks | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0
Redeemable Preferred Stocks | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	6,700,000
Collateralized Loan Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	618,200,000	524,000,000.0
Collateralized Loan Obligations | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	618,200,000	524,000,000.0
Collateralized Loan Obligations | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0	0
Collateralized Loan Obligations | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0	19,100,000
Collateralized Loan Obligations | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	618,200,000	504,900,000
Other Mortgage- and Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	89,000,000.0	15,700,000
Other Mortgage- and Asset-backed | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	89,000,000.0	15,700,000
Other Mortgage- and Asset-backed | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0	0
Other Mortgage- and Asset-backed | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	78,800,000	5,800,000
Other Mortgage- and Asset-backed | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	10,200,000	9,900,000
Dividends on Equity Securities Excluding Alternative Investments | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	907,300,000	684,400,000
Dividends on Equity Securities Excluding Alternative Investments | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	600,700,000	437,700,000
Dividends on Equity Securities Excluding Alternative Investments | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	58,500,000	54,700,000
Dividends on Equity Securities Excluding Alternative Investments | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	0	0
Preferred Stocks | Finance, Insurance and Real Estate | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	44,500,000	41,200,000
Preferred Stocks | Finance, Insurance and Real Estate | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	0	0
Preferred Stocks | Finance, Insurance and Real Estate | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	44,500,000	41,200,000
Preferred Stocks | Finance, Insurance and Real Estate | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	0	0
Preferred Stocks | Other Industries | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	14,700,000	13,000,000.0
Preferred Stocks | Other Industries | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	900,000	0
Preferred Stocks | Other Industries | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	13,800,000	13,000,000.0
Preferred Stocks | Other Industries | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	0	0
Common Stock | Finance, Insurance and Real Estate | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	12,800,000	10,200,000
Common Stock | Finance, Insurance and Real Estate | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	12,800,000	10,200,000
Common Stock | Finance, Insurance and Real Estate | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	0	0
Common Stock | Finance, Insurance and Real Estate | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	0	0
Common Stock | Other Industries | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	400,000	700,000
Common Stock | Other Industries | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	200,000	200,000
Common Stock | Other Industries | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	200,000	500,000
Common Stock | Other Industries | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	0	0
Bond Exchange Traded Funds | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	586,800,000	427,300,000
Bond Exchange Traded Funds | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	586,800,000	427,300,000
Bond Exchange Traded Funds | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	0	0
Bond Exchange Traded Funds | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	0	0
Limited Liability Companies and Limited Partnerships | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	248,100,000	192,000,000.0
Limited Liability Companies and Limited Partnerships | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	0	0
Limited Liability Companies and Limited Partnerships | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	0	0
Limited Liability Companies and Limited Partnerships | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	0	0
Measured at Net Asset Value | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible securities at fair value	0	0
Total	248,100,000	192,000,000.0
Measured at Net Asset Value | Investments in Fixed Maturities | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0	0
Measured at Net Asset Value | United States Government and Government Agencies and Authorities | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0	0
Measured at Net Asset Value | States and Political Subdivisions | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0	0
Measured at Net Asset Value | Foreign Governments | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0	0
Measured at Net Asset Value | Corporate Bonds and Notes | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0	0
Measured at Net Asset Value | Redeemable Preferred Stocks | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0
Measured at Net Asset Value | Collateralized Loan Obligations | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0	0
Measured at Net Asset Value | Other Mortgage- and Asset-backed | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5)	0	0
Measured at Net Asset Value | Dividends on Equity Securities Excluding Alternative Investments | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	248,100,000	192,000,000.0
Measured at Net Asset Value | Preferred Stocks | Finance, Insurance and Real Estate | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	0	0
Measured at Net Asset Value | Preferred Stocks | Other Industries | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	0	0
Measured at Net Asset Value | Common Stock | Finance, Insurance and Real Estate | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	0	0
Measured at Net Asset Value | Common Stock | Other Industries | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	0	0
Measured at Net Asset Value | Bond Exchange Traded Funds | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	0	0
Measured at Net Asset Value | Limited Liability Companies and Limited Partnerships | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8)	$ 248,100,000	$ 192,000,000.0