Investments Components of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Net Investment Income [Line Items]
Gross investment income	$ 261.3	$ 192.1	$ 514.2	$ 358.4
Investment expenses	(6.2)	(6.2)	(12.4)	(12.2)
Net Investment Income	255.1	185.9	501.8	346.2
Total equity securities
Net Investment Income [Line Items]
Gross investment income	29.9	24.6	58.6	50.5
Total equity securities | Nonredeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	15.9	10.4	31.4	21.3
Total equity securities | Common equities
Net Investment Income [Line Items]
Gross investment income	14.0	14.2	27.2	29.2
Available-for-sale Securities
Net Investment Income [Line Items]
Gross investment income	231.4	167.5	455.6	307.9
Short-term investments
Net Investment Income [Line Items]
Gross investment income	11.2	14.9	27.2	25.0
Total fixed maturities
Net Investment Income [Line Items]
Gross investment income	220.2	152.6	428.4	282.9
Total fixed maturities | U.S. government obligations
Net Investment Income [Line Items]
Gross investment income	69.2	45.9	122.9	85.7
Total fixed maturities | State and local government obligations
Net Investment Income [Line Items]
Gross investment income	8.9	9.1	18.2	19.1
Total fixed maturities | Corporate debt securities
Net Investment Income [Line Items]
Gross investment income	67.5	51.8	144.7	88.0
Total fixed maturities | Residential mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	4.6	6.6	11.2	13.5
Total fixed maturities | Commercial mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	33.4	20.2	65.1	41.4
Total fixed maturities | Other asset-backed securities
Net Investment Income [Line Items]
Gross investment income	28.1	16.1	54.1	29.7
Total fixed maturities | Redeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	$ 8.5	$ 2.9	$ 12.2	$ 5.5