Investments - Aging of Unrealized Losses on Investments in Fixed Maturities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
United States Government and Government Agencies and Authorities
Fair Value
Less Than 12 Months	$ 118.5	$ 401.1
12 Months or Longer	5.1	79.0
Total	123.6	480.1
Unrealized Losses
Less Than 12 Months	(1.3)	(7.6)
12 Months or Longer	0.0	(7.4)
Total	(1.3)	(15.0)
States and Political Subdivisions
Fair Value
Less Than 12 Months	63.0	299.4
12 Months or Longer	5.4	102.6
Total	68.4	402.0
Unrealized Losses
Less Than 12 Months	(0.7)	(5.0)
12 Months or Longer	(0.4)	(3.6)
Total	(1.1)	(8.6)
Foreign Governments
Fair Value
Less Than 12 Months	1.0	4.9
12 Months or Longer	3.1	0.0
Total	4.1	4.9
Unrealized Losses
Less Than 12 Months	(0.3)	(0.6)
12 Months or Longer	(1.3)	0.0
Total	(1.6)	(0.6)
Corporate Bonds and Notes
Fair Value
Less Than 12 Months	160.0	1,326.0
12 Months or Longer	70.7	116.8
Total	230.7	1,442.8
Unrealized Losses
Less Than 12 Months	(2.1)	(38.2)
12 Months or Longer	(5.0)	(10.9)
Total	(7.1)	(49.1)
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	95.5	439.2
12 Months or Longer	355.6	0.0
Total	451.1	439.2
Unrealized Losses
Less Than 12 Months	(1.9)	(13.2)
12 Months or Longer	(6.6)	0.0
Total	(8.5)	(13.2)
Other Mortgage- and Asset-backed
Fair Value
Less Than 12 Months	72.8	0.2
12 Months or Longer	0.0	4.5
Total	72.8	4.7
Unrealized Losses
Less Than 12 Months	(1.1)	0.0
12 Months or Longer	0.0	(0.1)
Total	(1.1)	(0.1)
Investments in Fixed Maturities
Fair Value
Less Than 12 Months	516.3	2,470.8
12 Months or Longer	439.9	302.9
Total	956.2	2,773.7
Unrealized Losses
Less Than 12 Months	(7.5)	(64.6)
12 Months or Longer	(13.3)	(22.0)
Total	$ (20.8)	$ (86.6)