Investments - Net Investment Income (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Net Investment Income [Line Items]
Total net investment income	$ 400.9	$ 422.5	$ 357.6
Investment Portfolio
Net Investment Income [Line Items]
Investment expenses	(10.5)	(10.1)	(9.6)
Total net investment income	297.4	321.4	260.3
Deposit Asset On Reinsurance
Net Investment Income [Line Items]
Total net investment income	103.5	101.1	97.3
Fixed maturity securities \| Investment Portfolio
Net Investment Income [Line Items]
Investment income	247.2	235.6	232.9
Equity securities \| Investment Portfolio
Net Investment Income [Line Items]
Investment income	9.0	5.3	4.7
Limited partnership interests \| Investment Portfolio
Net Investment Income [Line Items]
Investment income	40.5	79.0	20.9
Short-term and other investments \| Investment Portfolio
Net Investment Income [Line Items]
Investment income	$ 11.2	$ 11.6	$ 11.4

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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