Investments - Schedule of Maturities of Fixed Maturity Securities Available-for-Sale (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Amortized Cost
Amortized Cost	$ 3,824.0
Fair Value
Fair Value	3,753.0	$ 4,028.9
Corporate, Municipal, U.S. Government and Other securities
Amortized Cost
Due in one year or less	42.2
Due after one year through five years	277.5
Due after five years through ten years	427.5
Due after ten years	2,252.5
Subtotal	2,999.7
Fair Value
Due in one year or less	42.2
Due after one year through five years	269.1
Due after five years through ten years	392.4
Due after ten years	2,347.4
Subtotal	3,051.1
Mortgage-backed securities
Amortized Cost
Amortized cost, without single maturity date	172.5
Fair Value
Fair value, without single maturity date	160.7
Asset-backed securities
Amortized Cost
Amortized cost, without single maturity date	651.8
Amortized Cost	651.8	592.6
Fair Value
Fair value, without single maturity date	541.2
Fair Value	$ 541.2	$ 577.8