Investments - Aging of Unrealized Losses on Investments in Fixed Maturities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
United States Government and Government Agencies and Authorities
Fair Value
Less Than 12 Months	$ 10.5	$ 118.5
12 Months or Longer	0.0	5.1
Total	10.5	123.6
Unrealized Losses
Less Than 12 Months	(0.1)	(1.3)
12 Months or Longer	0.0	0.0
Total	(0.1)	(1.3)
States and Political Subdivisions
Fair Value
Less Than 12 Months	23.3	63.0
12 Months or Longer	0.0	5.4
Total	23.3	68.4
Unrealized Losses
Less Than 12 Months	(0.2)	(0.7)
12 Months or Longer	0.0	(0.4)
Total	(0.2)	(1.1)
Foreign Governments
Fair Value
Less Than 12 Months	0.5	1.0
12 Months or Longer	2.6	3.1
Total	3.1	4.1
Unrealized Losses
Less Than 12 Months	(0.1)	(0.3)
12 Months or Longer	(1.0)	(1.3)
Total	(1.1)	(1.6)
Corporate Bonds and Notes
Fair Value
Less Than 12 Months	132.9	160.0
12 Months or Longer	46.1	70.7
Total	179.0	230.7
Unrealized Losses
Less Than 12 Months	(7.5)	(2.1)
12 Months or Longer	(3.1)	(5.0)
Total	(10.6)	(7.1)
Redeemable Preferred Stocks
Fair Value
Less Than 12 Months		5.5
12 Months or Longer		0.0
Total		5.5
Unrealized Losses
Less Than 12 Months		(0.1)
12 Months or Longer		0.0
Total		(0.1)
Collateralized Loan Obligations
Fair Value
Less Than 12 Months	145.2	95.5
12 Months or Longer	371.4	355.6
Total	516.6	451.1
Unrealized Losses
Less Than 12 Months	(3.8)	(1.9)
12 Months or Longer	(15.9)	(6.6)
Total	(19.7)	(8.5)
Other Mortgage- and Asset-backed
Fair Value
Less Than 12 Months	6.3	72.8
12 Months or Longer	0.0	0.0
Total	6.3	72.8
Unrealized Losses
Less Than 12 Months	0.0	(1.1)
12 Months or Longer	0.0	0.0
Total	0.0	(1.1)
Investments in Fixed Maturities
Fair Value
Less Than 12 Months	318.7	516.3
12 Months or Longer	420.1	439.9
Total	738.8	956.2
Unrealized Losses
Less Than 12 Months	(11.7)	(7.5)
12 Months or Longer	(20.0)	(13.3)
Total	$ (31.7)	$ (20.8)