Investments - Schedule of Fixed Maturities (Details) - USD ($)	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	$ 7,222,500,000	$ 6,692,700,000
Gross unrealized gains	701,900,000
Gross unrealized loss	(34,900,000)
Fixed maturities, allowance for credit losses	(4,400,000)	(3,300,000)	$ (4,800,000)	$ 0
Fixed maturities at fair value	7,885,100,000	7,605,900,000
Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost		6,692,700,000
Gross unrealized gains		948,200,000
Gross unrealized loss		(31,700,000)
Fixed maturities at fair value		7,605,900,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss	34,900,000	31,700,000
Investments in Fixed Maturities | Investment-grade
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	27,200,000	8,000,000
U.S. Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	472,000,000.0	536,500,000
Gross unrealized gains	31,400,000	48,900,000
Gross unrealized loss	(800,000)	(100,000)
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	502,600,000	585,300,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss	800,000	100,000
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	1,780,300,000	1,404,300,000
Gross unrealized gains	148,800,000	185,400,000
Gross unrealized loss	(11,800,000)	(200,000)
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	1,917,300,000	1,589,500,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss	11,800,000	200,000
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	6,700,000	6,600,000
Gross unrealized gains	100,000	0
Gross unrealized loss	(1,100,000)	(1,100,000)
Fixed maturities, allowance for credit losses	0	(300,000)	(600,000)	0
Fixed maturities at fair value	5,700,000	5,200,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss	1,100,000	1,100,000
Bonds and Notes
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	3,948,800,000	3,749,500,000
Gross unrealized gains	504,700,000	689,500,000
Gross unrealized loss	(15,800,000)	(10,600,000)
Fixed maturities, allowance for credit losses	(4,400,000)	(3,000,000.0)	$ (4,200,000)	$ 0
Fixed maturities at fair value	4,433,300,000	4,425,400,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss	15,800,000	10,600,000
Redeemable Preferred Stocks
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	7,000,000.0	7,000,000.0
Gross unrealized gains	400,000	500,000
Gross unrealized loss	0	0
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	7,400,000	7,500,000
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	738,600,000	785,100,000
Gross unrealized gains	2,000,000.0	2,300,000
Gross unrealized loss	(4,700,000)	(19,700,000)
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	735,900,000	767,700,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss	4,700,000	19,700,000
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	269,100,000	203,700,000
Gross unrealized gains	14,500,000	21,600,000
Gross unrealized loss	(700,000)	0
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	282,900,000	225,300,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss	$ 700,000	$ 0