Investments (Continuous unrealized loss) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
U.S. Government and Government Agencies and Authorities
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months	$ 428.7	$ 140.0
12 Months or Longer	80.9	103.4
Total	509.6	243.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
12 Months or Longer	7.8	5.1
Less Than 12 Months	8.1	1.1
Total	15.9	6.2
States and Political Subdivisions
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months	213.6	57.0
12 Months or Longer	98.9	124.2
Total	312.5	181.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
12 Months or Longer	4.1	2.0
Less Than 12 Months	4.2	0.3
Total	8.3	2.3
Foreign Governments
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months	0.6
12 Months or Longer	0.0
Total	0.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
12 Months or Longer	0.0
Less Than 12 Months	0.0
Total	0.0
Bonds and Notes
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months	716.3	283.1
12 Months or Longer	146.8	208.9
Total	863.1	492.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
12 Months or Longer	7.6	6.8
Less Than 12 Months	16.1	4.2
Total	23.7	11.0
Collateralized Loan Obligations
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months	39.5	2.8
12 Months or Longer	0.0	2.4
Total	39.5	5.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
12 Months or Longer	0.0	0.0
Less Than 12 Months	0.1	0.0
Total	0.1	0.0
Other Mortgage- and Asset-backed
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months	4.5
12 Months or Longer	0.0
Total	4.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
12 Months or Longer	0.0
Less Than 12 Months	0.0
Total	0.0
Investments in Fixed Maturities
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months	1,403.2	482.9
12 Months or Longer	326.6	438.9
Total	1,729.8	921.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
12 Months or Longer	19.5	13.9
Less Than 12 Months	28.5	5.6
Total	$ 48.0	$ 19.5