Investment Securities (Net Investment Income) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Investment income
Total investment income	$ 23.2	$ 16.4
Third-party investment expenses	(0.3)	(0.4)
Net investment income, pre-tax	22.9	16.0
Fixed maturity investments
Investment income
Total investment income	8.1	8.1
Short-term investments
Investment income
Total investment income	0.8	1.3
Common Stock
Investment income
Total investment income	3.8	5.4
Other long-term investments
Investment income
Total investment income	$ 10.5	$ 1.6

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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Amount of expenses related to the generation of investment income.

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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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