Investments Fixed Maturity Securities Stratified by Contractual Maturity (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Investments [Abstract]
Available for sale, Due in one year or less (amortized cost)	$ 1,029.6
Available for sale, Due after one year through five years (amortized cost)	4,914.0
Available for sale, Due after five years through ten years (amortized cost)	2,423.3
Available for sale, Due after ten years (amortized cost)	42.1
Available for sale, Fixed maturity securities (amortized cost)	8,409.1	$ 8,537.3
Available for sale, Due in one year or less (estimated fair value)	1,027.7
Available for sale, Due after one year through five years (estimated fair value)	4,961.4
Available for sale, Due after five years through ten years (estimated fair value)	2,465.1
Available for sale, Due after ten years (estimated fair value)	41.1
Available for sale, Fixed maturity securities (estimated fair value)	8,495.4	8,796.5
Held to maturity, Due within one year or less (amortized cost)	1.0
Held to maturity, Due after one year through five years (amortized cost)	459.4
Held to maturity, Due after five years through then years (amortized cost)	555.3
Held to maturity, Due after ten years (amortized cost)	0.0
Held to maturity (amortized cost)	1,015.8	1,021.7
Held to maturity, Due in one year or less (estimated fair value)	1.0
Held to maturity, Due after one year through five years (estimated fair value)	469.6
Held to maturity, Due after five years through ten years (estimated fair value)	577.5
Held to maturity, Due after ten years (estimated fair value)	0.0
Held to maturity (estimated fair value)	$ 1,048.5	$ 1,058.2