Investments Fixed Maturity Securities by Type (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Available for sale, Fixed maturity securities (amortized cost)	$ 8,409.1	$ 8,537.3
Available for sale, Fixed maturity securities (estimated fair value)	8,495.4	8,796.5
Held to maturity (estimated fair value)	1,048.5	1,058.2
U.S. & Canadian Governments [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Available for sale, Fixed maturity securities (amortized cost)	1,752.3	1,842.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	108.1	36.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.0	0.4
Available for sale, Fixed maturity securities (estimated fair value)	1,860.4	1,878.8
Tax-exempt [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment	1,015.8	1,021.7
Held to maturity (gross unrealized gains)	32.7	36.5
Held to maturity (gross unrealized losses)	0.0	0.0
Held to maturity (estimated fair value)	1,048.5	1,058.2
Corporate [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Available for sale, Fixed maturity securities (amortized cost)	6,656.8	6,694.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	135.1	225.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	157.0	2.8
Available for sale, Fixed maturity securities (estimated fair value)	6,634.9	6,917.6
Fixed Maturity Securities [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Available for sale, Fixed maturity securities (amortized cost)	8,409.1	8,537.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	243.3	262.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	157.0	3.3
Available for sale, Fixed maturity securities (estimated fair value)	$ 8,495.4	$ 8,796.5

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