Investments - Schedule of Fixed Maturities (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	$ 6,667,300,000	$ 6,372,700,000
Gross unrealized gains	515,700,000
Gross unrealized loss	(179,900,000)
Fixed maturities, allowance for credit losses	(4,600,000)	0
Fixed maturities at fair value	6,998,500,000	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	700,300,000	784,700,000
Gross unrealized gains	60,300,000	32,500,000
Gross unrealized loss	0	(1,300,000)
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	760,600,000	815,900,000
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	1,400,100,000	1,386,400,000
Gross unrealized gains	133,000,000.0	130,500,000
Gross unrealized loss	(4,100,000)	(1,100,000)
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	1,529,000,000.0	1,515,800,000
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	7,300,000	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	(1,100,000)	0
Fixed maturities at fair value	5,000,000.0	16,800,000
Bonds and Notes
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	3,697,700,000	3,465,000,000.0
Gross unrealized gains	314,400,000	401,800,000
Gross unrealized loss	(74,600,000)	(7,100,000)
Fixed maturities, allowance for credit losses	(3,500,000)	0
Fixed maturities at fair value	3,934,000,000.0	3,859,700,000
Redeemable Preferred Stocks
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	6,800,000	6,800,000
Gross unrealized gains	0	0
Gross unrealized loss	(100,000)	(100,000)
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	6,700,000	6,700,000
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	684,500,000	624,600,000
Gross unrealized gains	800,000	2,100,000
Gross unrealized loss	(91,800,000)	(8,500,000)
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	593,500,000	618,200,000
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Investments in Fixed Maturities	170,600,000	88,000,000.0
Gross unrealized gains	7,200,000	2,100,000
Gross unrealized loss	(8,100,000)	(1,100,000)
Fixed maturities, allowance for credit losses	0
Fixed maturities at fair value	$ 169,700,000	$ 89,000,000.0