Investments Components of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Net Investment Income [Line Items]
Gross investment income	$ 220.2	$ 241.2
Investment expenses	(5.6)	(5.3)
Net Investment Income	214.6	235.9
Available-for-sale Securities
Net Investment Income [Line Items]
Gross investment income	188.1	212.8
Total equity securities
Net Investment Income [Line Items]
Gross investment income	32.1	28.4
Total equity securities | Nonredeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	17.9	13.8
Total equity securities | Common equities
Net Investment Income [Line Items]
Gross investment income	14.2	14.6
Total fixed maturities
Net Investment Income [Line Items]
Gross investment income	186.6	205.8
Total fixed maturities | U.S. government obligations
Net Investment Income [Line Items]
Gross investment income	30.4	62.6
Total fixed maturities | State and local government obligations
Net Investment Income [Line Items]
Gross investment income	13.4	9.6
Total fixed maturities | Corporate debt securities
Net Investment Income [Line Items]
Gross investment income	85.2	58.0
Total fixed maturities | Residential mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	3.4	4.2
Total fixed maturities | Commercial mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	35.8	34.3
Total fixed maturities | Other asset-backed securities
Net Investment Income [Line Items]
Gross investment income	15.9	29.2
Total fixed maturities | Redeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	2.5	7.9
Short-term investments
Net Investment Income [Line Items]
Gross investment income	$ 1.5	$ 7.0