Income from Investments - Net Realized Gains on Sales of Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Equity Securities:
Realized Investment Gains	$ 6.7	$ 41.3		$ 68.0
Realized Investment Losses	(13.4)	(38.7)		(3.2)
Net Realized Investment (Losses) Gains	(18.6)	4.3	[1]	64.8	[1]
Fixed Maturities:
Fixed Maturities:
Gains on Sales	5.9	31.6		63.4
Losses on Sales	(10.9)	(31.9)		(2.1)
Equity Securities
Equity Securities:
Gains on Sales	0.6	9.7		4.1
Losses on Sales	(2.5)	(6.8)		(0.7)
Real Estate
Equity Securities:
Realized Investment Gains	0.0	0.0		0.1
Realized Investment Losses	0.0	0.0		(0.4)
Derivative
Equity Securities:
(Losses) Gains on Hedging Activity	(11.9)	1.7		0.0
Other Long-Term Investments
Equity Securities:
Realized Investment Gains	$ 0.2	$ 0.0		$ 0.0

[1] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.