Investments Fixed Maturity Securities by Type (Details) - USD ($) $ in Millions	Jun. 30, 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)	$ 9,305.2	$ 8,537.3
Available for sale, Fixed maturity securities (estimated fair value)	9,859.4	8,796.5
Held to maturity (estimated fair value)	0.0	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)	1,712.5	1,842.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	109.2	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)	1,821.8	1,878.8
Tax-exempt [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Available for sale, Fixed maturity securities (amortized cost)	1,010.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	61.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.0
Available for sale, Fixed maturity securities (estimated fair value)	1,071.4
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment		1,021.7
Held to maturity (gross unrealized gains)		36.5
Held to maturity (gross unrealized losses)		0.0
Held to maturity (estimated fair value)		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)	6,582.7	6,694.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	403.8	225.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	20.3	2.8
Available for sale, Fixed maturity securities (estimated fair value)	6,966.2	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)	9,305.2	8,537.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	574.5	262.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	20.3	3.3
Available for sale, Fixed maturity securities (estimated fair value)	$ 9,859.4	$ 8,796.5

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