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Investments - Summary of Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 50.5	$ 49.5	$ 153.1	$ 150.3
Investment expenses	(2.5)	(3.1)	(7.4)	(8.8)
Net investment income	48.0	46.4	145.7	141.5
Total fixed income securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	33.0	32.9	99.8	100.7
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.4	0.1	0.9	0.4
Short-term Investment, Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.1	0.0	0.4	0.5
Fixed term deposits (included in cash and cash equivalents)
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	3.5	2.1	10.1	4.1
Equity securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.0	2.8	1.9	11.7
Catastrophe bonds — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.5	0.5	1.8	1.3
Other investments, at fair value
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.5	0.0	0.5	0.0
Fixed income maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 12.5	$ 11.1	$ 37.7	$ 31.6

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
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Amount of expenses related to the generation of investment income.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
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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.

+ References

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