Investments - Continuous Unrealized Loss (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
U.S. Government and Government Agencies and Authorities
Fair Value
Less Than 12 Months	$ 22.0	$ 401.1
12 Months or Longer	5.6	79.0
Total	27.6	480.1
Unrealized Losses
Less Than 12 Months	(0.2)	(7.6)
12 Months or Longer	(0.1)	(7.4)
Total	(0.3)	(15.0)
States and Political Subdivisions
Fair Value
Less Than 12 Months	22.7	299.4
12 Months or Longer	6.6	102.6
Total	29.3	402.0
Unrealized Losses
Less Than 12 Months	(0.3)	(5.0)
12 Months or Longer	(0.3)	(3.6)
Total	(0.6)	(8.6)
Foreign Governments
Fair Value
Less Than 12 Months	1.1	4.9
12 Months or Longer	2.4	0.0
Total	3.5	4.9
Unrealized Losses
Less Than 12 Months	(0.5)	(0.6)
12 Months or Longer	(2.0)	0.0
Total	(2.5)	(0.6)
Bonds and Notes
Fair Value
Less Than 12 Months	200.5	1,326.0
12 Months or Longer	71.7	116.8
Total	272.2	1,442.8
Unrealized Losses
Less Than 12 Months	(3.3)	(38.2)
12 Months or Longer	(6.5)	(10.9)
Total	(9.8)	(49.1)
Collateralized Loan Obligations
Fair Value
Less Than 12 Months	353.4	439.2
12 Months or Longer	87.4	0.0
Total	440.8	439.2
Unrealized Losses
Less Than 12 Months	(5.4)	(13.2)
12 Months or Longer	(3.0)	0.0
Total	(8.4)	(13.2)
Other Mortgage- and Asset-backed
Fair Value
Less Than 12 Months		0.2
12 Months or Longer		4.5
Total		4.7
Unrealized Losses
Less Than 12 Months		0.0
12 Months or Longer		(0.1)
Total		(0.1)
Investments in Fixed Maturities
Fair Value
Less Than 12 Months	599.7	2,470.8
12 Months or Longer	173.7	302.9
Total	773.4	2,773.7
Unrealized Losses
Less Than 12 Months	(9.7)	(64.6)
12 Months or Longer	(11.9)	(22.0)
Total	$ (21.6)	$ (86.6)