Investments - Continuous Unrealized Loss (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
U.S. Government and Government Agencies and Authorities
Fair Value
Less Than 12 Months		$ 118.5
12 Months or Longer		5.1
Total		123.6
Unrealized Losses
Less Than 12 Months		(1.3)
12 Months or Longer		0.0
Total		(1.3)
States and Political Subdivisions
Fair Value
Less Than 12 Months	$ 66.0	63.0
12 Months or Longer	1.6	5.4
Total	67.6	68.4
Unrealized Losses
Less Than 12 Months	(3.9)	(0.7)
12 Months or Longer	(0.2)	(0.4)
Total	(4.1)	(1.1)
Foreign Governments
Fair Value
Less Than 12 Months	2.9	1.0
12 Months or Longer	2.1	3.1
Total	5.0	4.1
Unrealized Losses
Less Than 12 Months	(0.5)	(0.3)
12 Months or Longer	(0.7)	(1.3)
Total	(1.2)	(1.6)
Bonds and Notes
Fair Value
Less Than 12 Months	888.0	160.0
12 Months or Longer	30.5	70.7
Total	918.5	230.7
Unrealized Losses
Less Than 12 Months	(68.7)	(2.1)
12 Months or Longer	(5.9)	(5.0)
Total	(74.6)	(7.1)
Redeemable Preferred Stocks
Fair Value
Less Than 12 Months	6.7	5.5
12 Months or Longer	0.0	0.0
Total	6.7	5.5
Unrealized Losses
Less Than 12 Months	(0.1)	(0.1)
12 Months or Longer	0.0	0.0
Total	(0.1)	(0.1)
Collateralized Loan Obligations
Fair Value
Less Than 12 Months	308.6	95.5
12 Months or Longer	253.3	355.6
Total	561.9	451.1
Unrealized Losses
Less Than 12 Months	(51.0)	(1.9)
12 Months or Longer	(40.8)	(6.6)
Total	(91.8)	(8.5)
Other Mortgage- and Asset-backed
Fair Value
Less Than 12 Months	76.8	72.8
12 Months or Longer	0.0	0.0
Total	76.8	72.8
Unrealized Losses
Less Than 12 Months	(8.1)	(1.1)
12 Months or Longer	0.0	0.0
Total	(8.1)	(1.1)
Investments in Fixed Maturities
Fair Value
Less Than 12 Months	1,349.0	516.3
12 Months or Longer	287.5	439.9
Total	1,636.5	956.2
Unrealized Losses
Less Than 12 Months	(132.3)	(7.5)
12 Months or Longer	(47.6)	(13.3)
Total	$ (179.9)	$ (20.8)