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Investments - Summary of Investment Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$ 209.1	$ 207.1	$ 200.1
Investment expenses	(11.8)	(8.9)	(11.1)
Net investment income	197.3	198.2	189.0
Fixed income securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	128.2	134.1	133.3
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	2.3	1.4	0.4
Short-term investment, Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	2.5	0.4	0.8
Fixed term deposits (included in cash and cash equivalents)
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	19.5	14.2	6.2
Equity securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	0.0	2.1	13.6
Catastrophe bonds — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	2.3	2.8	1.8
Privately-held investments — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	3.4	0.0	0.0
Other investments, at fair value
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	8.9	2.5	0.0
Fixed Income Securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$ 42.0	$ 49.6	$ 44.0

X

- Definition

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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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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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