Investments Gross Unrealized Losses and Fair Value (Details) $ in Millions	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Available for sale, Fair Value, Less than 12 Months	$ 278.1	$ 393.7
Available for sale, Unrealized Losses, Less than 12 Months	16.5	2.3
Available for sale, Fair Value, 12 Months or Greater	16.9	107.4
Available for sale, Unrealized Losses, 12 Months or Greater	3.8	1.0
Available for sale, Fair Value, Total	295.1	501.1
Available for sale, Unrealized Losses, Total	20.3	3.3
U.S. & Canadian Governments [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Available for sale, Fair Value, Less than 12 Months	0.0	217.2
Available for sale, Unrealized Losses, Less than 12 Months	0.0	0.3
Available for sale, Fair Value, 12 Months or Greater	0.0	53.0
Available for sale, Unrealized Losses, 12 Months or Greater	0.0	0.1
Available for sale, Fair Value, Total	0.0	270.3
Available for sale, Unrealized Losses, Total	0.0	0.4
Tax-exempt [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Available for sale, Fair Value, Less than 12 Months	0.0
Available for sale, Unrealized Losses, Less than 12 Months	0.0
Available for sale, Fair Value, 12 Months or Greater	0.0
Available for sale, Unrealized Losses, 12 Months or Greater	0.0
Available for sale, Fair Value, Total	0.0
Available for sale, Unrealized Losses, Total	0.0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value		0.0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0.0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value		21.7
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0.0
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value		21.7
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss		$ 0.0
Held-to-maturity Securities [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Number of securities in continuous unrealized loss position for less than 12 months		0
Number of securities in continuous unrealized loss position for 12 months or greater		8
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions		8
Corporate [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Available for sale, Fair Value, Less than 12 Months	278.1	$ 176.4
Available for sale, Unrealized Losses, Less than 12 Months	16.5	1.9
Available for sale, Fair Value, 12 Months or Greater	16.8	54.3
Available for sale, Unrealized Losses, 12 Months or Greater	3.8	0.8
Available for sale, Fair Value, Total	295.0	230.7
Available for sale, Unrealized Losses, Total	$ 20.3	$ 2.8
Available-for-sale Securities [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Number of securities in continuous unrealized loss position for less than 12 months	46	54
Number of securities in continuous unrealized loss position for 12 months or greater	12	47
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	58	101