Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Net Investment Income [Line Items]
Investment expenses	$ (2.1)	$ (2.2)
Total net investment income	95.5	82.3
Investment Portfolio
Net Investment Income [Line Items]
Total net investment income	71.1	58.6
Deposit asset on reinsurance
Net Investment Income [Line Items]
Total net investment income	24.4	23.7
Fixed maturity securities
Net Investment Income [Line Items]
Net investment income - investment portfolio	58.0	59.4
Equity securities
Net Investment Income [Line Items]
Net investment income - investment portfolio	1.1	1.2
Limited partnership interests
Net Investment Income [Line Items]
Net investment income - investment portfolio	11.3	(2.7)
Short-term and other investments
Net Investment Income [Line Items]
Net investment income - investment portfolio	$ 2.8	$ 2.9

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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