Investments - Amortized Cost and Estimated Fair Value of investments (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	$ 6,692,700,000	$ 6,372,700,000
Gross Unrealized Gains	948,200,000	570,200,000
Gross Unrealized Losses	(31,700,000)	(20,800,000)
Fair Value	7,605,900,000	6,922,100,000
Amortized cost of fixed maturities	(3,300,000)	0
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	536,500,000	784,700,000
Gross Unrealized Gains	48,900,000	32,500,000
Gross Unrealized Losses	(100,000)	(1,300,000)
Fair Value	585,300,000	815,900,000
Amortized cost of fixed maturities	0
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	1,404,300,000	1,386,400,000
Gross Unrealized Gains	185,400,000	130,500,000
Gross Unrealized Losses	(200,000)	(1,100,000)
Fair Value	1,589,500,000	1,515,800,000
Amortized cost of fixed maturities	0
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	6,600,000	17,200,000
Gross Unrealized Gains	0	1,200,000
Gross Unrealized Losses	(1,100,000)	(1,600,000)
Fair Value	5,200,000	16,800,000
Amortized cost of fixed maturities	(300,000)	0
Corporate Bonds and Notes
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	3,749,500,000	3,465,000,000.0
Gross Unrealized Gains	689,500,000	401,800,000
Gross Unrealized Losses	(10,600,000)	(7,100,000)
Fair Value	4,425,400,000	3,859,700,000
Amortized cost of fixed maturities	(3,000,000.0)	0
Redeemable Preferred Stocks
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	7,000,000.0	6,800,000
Gross Unrealized Gains	500,000	0
Gross Unrealized Losses	0	(100,000)
Fair Value	7,500,000	6,700,000
Amortized cost of fixed maturities	0
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	785,100,000	624,600,000
Gross Unrealized Gains	2,300,000	2,100,000
Gross Unrealized Losses	(19,700,000)	(8,500,000)
Fair Value	767,700,000	618,200,000
Amortized cost of fixed maturities	0
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	203,700,000	88,000,000.0
Gross Unrealized Gains	21,600,000	2,100,000
Gross Unrealized Losses	0	(1,100,000)
Fair Value	225,300,000	$ 89,000,000.0
Amortized cost of fixed maturities	$ 0