Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Net Investment Income [Line Items]
Investment expenses	$ (2.5)	$ (2.9)	$ (4.6)	$ (5.1)
Total net investment income	109.2	80.4	204.7	162.7
Investment Portfolio
Net Investment Income [Line Items]
Total net investment income	84.1	56.5	155.2	115.1
Deposit asset on reinsurance
Net Investment Income [Line Items]
Total net investment income	25.1	23.9	49.5	47.6
Fixed maturity securities
Net Investment Income [Line Items]
Net investment income - investment portfolio	59.4	58.9	117.4	118.3
Equity securities
Net Investment Income [Line Items]
Net investment income - investment portfolio	1.3	1.2	2.4	2.4
Limited partnership interests
Net Investment Income [Line Items]
Net investment income - investment portfolio	23.0	(3.5)	34.3	(6.2)
Short-term and other investments
Net Investment Income [Line Items]
Net investment income - investment portfolio	$ 2.9	$ 2.8	$ 5.7	$ 5.7

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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