Investments - Schedule of Fixed Maturities (Details) - USD ($)	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	$ 7,092,600,000	$ 6,692,700,000
Gross unrealized gains	767,500,000
Gross unrealized loss	(20,000,000.0)
Fixed maturities, allowance for credit losses	(5,100,000)	(3,300,000)	$ (5,400,000)	$ 0
Fixed maturities at fair value	7,835,000,000.0	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	20,000,000.0	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	11,500,000	8,000,000.0
U.S. Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	485,800,000	536,500,000
Gross unrealized gains	37,100,000	48,900,000
Gross unrealized loss	(600,000)	(100,000)
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	522,300,000	585,300,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss	600,000	100,000
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	1,640,600,000	1,404,300,000
Gross unrealized gains	167,500,000	185,400,000
Gross unrealized loss	(2,300,000)	(200,000)
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	1,805,800,000	1,589,500,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss	2,300,000	200,000
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	6,300,000	6,600,000
Gross unrealized gains	100,000	0
Gross unrealized loss	(1,300,000)	(1,100,000)
Fixed maturities, allowance for credit losses	0	(300,000)	(600,000)	0
Fixed maturities at fair value	5,100,000	5,200,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss	1,200,000	1,100,000
Bonds and Notes
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	3,947,200,000	3,749,500,000
Gross unrealized gains	543,700,000	689,500,000
Gross unrealized loss	(10,400,000)	(10,600,000)
Fixed maturities, allowance for credit losses	(5,100,000)	(3,000,000.0)	$ (4,800,000)	$ 0
Fixed maturities at fair value	4,475,400,000	4,425,400,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss	10,500,000	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	500,000	500,000
Gross unrealized loss	0	0
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	7,500,000	7,500,000
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	773,100,000	785,100,000
Gross unrealized gains	2,400,000	2,300,000
Gross unrealized loss	(5,300,000)	(19,700,000)
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	770,200,000	767,700,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss	5,300,000	19,700,000
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	232,600,000	203,700,000
Gross unrealized gains	16,200,000	21,600,000
Gross unrealized loss	(100,000)	0
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	248,700,000	225,300,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss	$ 100,000	$ 0