Investments (Continuous unrealized loss) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
U.S. Government and Government Agencies and Authorities
Debt Securities, Available-for-sale, Unrealized Loss Position [Abstract]
12 Months or Longer	$ 262.2	$ 79.0
Total	275.4	480.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	13.2	401.1
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Less Than 12 Months	0.2	7.6
12 Months or Longer	7.5	7.4
Total	7.7	15.0
States and Political Subdivisions
Debt Securities, Available-for-sale, Unrealized Loss Position [Abstract]
12 Months or Longer	77.8	102.6
Total	79.4	402.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1.6	299.4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Less Than 12 Months	0.3	5.0
12 Months or Longer	1.4	3.6
Total	1.7	8.6
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		4.9
Debt Securities, Available-for-sale, Unrealized Loss Position [Abstract]
12 Months or Longer	0.5
Total	4.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		4.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	4.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Less Than 12 Months	0.5
12 Months or Longer	0.1
Total	0.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		0.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0.0
Bonds and Notes
Debt Securities, Available-for-sale, Unrealized Loss Position [Abstract]
12 Months or Longer	321.4	116.8
Total	544.0	1,442.8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	222.6	1,326.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Less Than 12 Months	5.2	38.2
12 Months or Longer	12.6	10.9
Total	17.8	49.1
Collateralized Loan Obligations
Debt Securities, Available-for-sale, Unrealized Loss Position [Abstract]
12 Months or Longer	28.7	0.0
Total	435.3	439.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	406.6	439.2
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Less Than 12 Months	6.0	13.2
12 Months or Longer	0.8	0.0
Total	6.8	13.2
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		4.7
Debt Securities, Available-for-sale, Unrealized Loss Position [Abstract]
12 Months or Longer	4.6
Total	4.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		0.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		4.5
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Less Than 12 Months	0.0
12 Months or Longer	0.0
Total	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		0.1
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0.1
Investments in Fixed Maturities
Debt Securities, Available-for-sale, Unrealized Loss Position [Abstract]
12 Months or Longer	695.2	302.9
Total	1,343.6	2,773.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	648.4	2,470.8
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Less Than 12 Months	12.2	64.6
12 Months or Longer	22.4	22.0
Total	34.6	86.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	34.6	86.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 22.4	$ 22.0