Investments Fixed Maturity Securities Stratified by Contractual Maturity (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Investments [Abstract]
Available for sale, Due in one year or less (amortized cost)	$ 1,089.0
Available for sale, Due after one year through five years (amortized cost)	5,345.3
Available for sale, Due after five years through ten years (amortized cost)	2,794.1
Available for sale, Due after ten years (amortized cost)	76.7
Available for sale, Fixed maturity securities (amortized cost)	9,305.2	$ 8,537.3
Available for sale, Due in one year or less (estimated fair value)	1,099.7
Available for sale, Due after one year through five years (estimated fair value)	5,620.7
Available for sale, Due after five years through ten years (estimated fair value)	3,059.2
Available for sale, Due after ten years (estimated fair value)	79.6
Available for sale, Fixed maturity securities (estimated fair value)	9,859.4	8,796.5
Held to maturity (amortized cost)	0.0	1,021.7
Held to maturity (estimated fair value)	$ 0.0	$ 1,058.2

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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