Investments - 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
Net Investment Income, Total	$ 103.7	$ 93.7	$ 308.4	$ 256.4
Investment Portfolio
Net Investment Income
Investment Income, Investment Expense	(2.7)	(2.1)	(7.3)	(7.2)
Net Investment Income, Total	78.1	69.2	233.3	184.3
Deposit asset on reinsurance
Net Investment Income
Net Investment Income, Total	25.6	24.5	75.1	72.1
Fixed maturity securities \| Investment Portfolio
Net Investment Income
Net investment income - investment portfolio	59.8	56.4	177.2	174.7
Equity securities \| Investment Portfolio
Net Investment Income
Net investment income - investment portfolio	1.4	1.1	3.8	3.5
Limited partnership interests \| Investment Portfolio
Net Investment Income
Net investment income - investment portfolio	16.8	11.1	51.1	4.9
Short-term and other investments \| Investment Portfolio
Net Investment Income
Net investment income - investment portfolio	$ 2.8	$ 2.7	$ 8.5	$ 8.4

Represents the total of interest and dividend income, including any amortization and accretion (as applicable) of discounts and premiums, earned from (1) loans and leases whether held-for-sale or held-in-portfolio; (2) investment securities; (3) federal funds sold; (4) securities purchased under agreements to resell; (5) investments in banker's acceptances, commercial paper, or certificates of deposit; (6) dividend income; or (7) other investments not otherwise specified herein.

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