Investments - Schedule of Fixed Maturities (Details) - USD ($)	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	$ 6,694,800,000		$ 6,372,700,000
Gross unrealized gains	870,000,000.0
Gross unrealized loss	(55,200,000)
Fixed maturities, allowance for credit losses	(4,800,000)	$ (5,400,000)	0
Fixed maturities at fair value	7,504,800,000		6,922,100,000
Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities			6,372,700,000
Gross unrealized gains			570,200,000
Gross unrealized loss			(20,800,000)
Fixed maturities at fair value			6,922,100,000
U.S. Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	596,600,000		784,700,000
Gross unrealized gains	56,200,000		32,500,000
Gross unrealized loss	0		(1,300,000)
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	652,800,000		815,900,000
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	1,334,300,000		1,386,400,000
Gross unrealized gains	178,900,000		130,500,000
Gross unrealized loss	(1,600,000)		(1,100,000)
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	1,511,600,000		1,515,800,000
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	6,700,000		17,200,000
Gross unrealized gains	0		1,200,000
Gross unrealized loss	(1,200,000)		(1,600,000)
Fixed maturities, allowance for credit losses	(600,000)	(600,000)	0
Fixed maturities at fair value	4,900,000		16,800,000
Bonds and Notes
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	3,786,800,000		3,465,000,000.0
Gross unrealized gains	611,600,000		401,800,000
Gross unrealized loss	(18,500,000)		(7,100,000)
Fixed maturities, allowance for credit losses	(4,200,000)	$ (4,800,000)	0
Fixed maturities at fair value	4,375,700,000		3,859,700,000
Redeemable Preferred Stocks
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	6,700,000		6,800,000
Gross unrealized gains	600,000		0
Gross unrealized loss	0		(100,000)
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	7,300,000		6,700,000
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	759,500,000		624,600,000
Gross unrealized gains	1,800,000		2,100,000
Gross unrealized loss	(33,900,000)		(8,500,000)
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	727,400,000		618,200,000
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	204,200,000		88,000,000.0
Gross unrealized gains	20,900,000		2,100,000
Gross unrealized loss	0		(1,100,000)
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	$ 225,100,000		$ 89,000,000.0