Investments Components of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Net Investment Income [Line Items]
Gross investment income	$ 208.9	$ 230.5	$ 639.8	$ 715.5
Investment expenses	(6.6)	(4.7)	(18.5)	(14.5)
Net Investment Income	202.3	225.8	621.3	701.0
Available-for-sale Securities
Net Investment Income [Line Items]
Gross investment income	175.9	201.2	541.4	629.4
Total equity securities
Net Investment Income [Line Items]
Gross investment income	33.0	29.3	98.4	86.1
Total equity securities | Nonredeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	17.3	15.8	52.5	44.3
Total equity securities | Common equities
Net Investment Income [Line Items]
Gross investment income	15.7	13.5	45.9	41.8
Total fixed maturities
Net Investment Income [Line Items]
Gross investment income	175.5	195.0	538.8	602.5
Total fixed maturities | U.S. government obligations
Net Investment Income [Line Items]
Gross investment income	39.0	36.7	106.8	135.0
Total fixed maturities | State and local government obligations
Net Investment Income [Line Items]
Gross investment income	9.7	18.5	34.6	44.2
Total fixed maturities | Corporate debt securities
Net Investment Income [Line Items]
Gross investment income	70.2	76.6	228.6	211.2
Total fixed maturities | Residential mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	3.2	0.8	9.4	8.8
Total fixed maturities | Commercial mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	35.0	37.3	105.2	114.2
Total fixed maturities | Other asset-backed securities
Net Investment Income [Line Items]
Gross investment income	16.1	22.6	47.1	76.6
Total fixed maturities | Redeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	2.3	2.5	7.1	12.5
Short-term investments
Net Investment Income [Line Items]
Gross investment income	$ 0.4	$ 6.2	$ 2.6	$ 26.9