Investments Components of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Net Investment Income [Line Items]
Gross investment income	$ 210.7	$ 243.8	$ 430.9	$ 485.0
Investment expenses	(6.3)	(4.5)	(11.9)	(9.8)
Net Investment Income	204.4	239.3	419.0	475.2
Available-for-sale Securities
Net Investment Income [Line Items]
Gross investment income	177.4	215.4	365.5	428.2
Total equity securities
Net Investment Income [Line Items]
Gross investment income	33.3	28.4	65.4	56.8
Total equity securities | Nonredeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	17.3	14.7	35.2	28.5
Total equity securities | Common equities
Net Investment Income [Line Items]
Gross investment income	16.0	13.7	30.2	28.3
Total fixed maturities
Net Investment Income [Line Items]
Gross investment income	176.7	201.7	363.3	407.5
Total fixed maturities | U.S. government obligations
Net Investment Income [Line Items]
Gross investment income	37.4	35.7	67.8	98.3
Total fixed maturities | State and local government obligations
Net Investment Income [Line Items]
Gross investment income	11.5	16.1	24.9	25.7
Total fixed maturities | Corporate debt securities
Net Investment Income [Line Items]
Gross investment income	73.2	76.6	158.4	134.6
Total fixed maturities | Residential mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	2.8	3.8	6.2	8.0
Total fixed maturities | Commercial mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	34.4	42.6	70.2	76.9
Total fixed maturities | Other asset-backed securities
Net Investment Income [Line Items]
Gross investment income	15.1	24.8	31.0	54.0
Total fixed maturities | Redeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	2.3	2.1	4.8	10.0
Short-term investments
Net Investment Income [Line Items]
Gross investment income	$ 0.7	$ 13.7	$ 2.2	$ 20.7