Investments Components of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Net Investment Income [Line Items]
Gross investment income	$ 263.0	$ 218.1	$ 777.2	$ 576.5
Investment expenses	(5.9)	(5.8)	(18.3)	(18.0)
Net Investment Income	257.1	212.3	758.9	558.5
Total equity securities
Net Investment Income [Line Items]
Gross investment income	30.0	28.1	88.6	78.6
Total equity securities | Nonredeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	15.9	11.2	47.3	32.5
Total equity securities | Common equities
Net Investment Income [Line Items]
Gross investment income	14.1	16.9	41.3	46.1
Available-for-sale Securities
Net Investment Income [Line Items]
Gross investment income	233.0	190.0	688.6	497.9
Short-term investments
Net Investment Income [Line Items]
Gross investment income	7.1	15.7	34.3	40.7
Total fixed maturities
Net Investment Income [Line Items]
Gross investment income	225.9	174.3	654.3	457.2
Total fixed maturities | U.S. government obligations
Net Investment Income [Line Items]
Gross investment income	75.8	47.7	198.7	133.4
Total fixed maturities | State and local government obligations
Net Investment Income [Line Items]
Gross investment income	8.7	9.0	26.9	28.1
Total fixed maturities | Corporate debt securities
Net Investment Income [Line Items]
Gross investment income	65.1	62.1	209.8	150.1
Total fixed maturities | Residential mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	5.4	6.9	16.6	20.4
Total fixed maturities | Commercial mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	35.6	24.1	100.7	65.5
Total fixed maturities | Other asset-backed securities
Net Investment Income [Line Items]
Gross investment income	31.4	21.2	85.5	50.9
Total fixed maturities | Redeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	$ 3.9	$ 3.3	$ 16.1	$ 8.8