Investment securities - Net Investment Income (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Investment Income
Interest on funds held under reinsurance treaties	$ (0.5)		$ (0.5)
Total investment income	82.7	$ 66.6	67.4
Investment expenses	(11.3)	(9.8)	(11.2)
Net investment income	71.4	56.8	56.2
Fixed maturity investments
Net Investment Income
Total investment income	53.2	51.5	58.0
Short-term investments
Net Investment Income
Total investment income	5.1	1.5	0.9
Equity securities
Net Investment Income
Total investment income	16.5	5.1	4.1
Other long-term investments
Net Investment Income
Total investment income	$ 8.4	$ 8.5	$ 4.9

Interest on funds held under reinsurance treaties.

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- Definition

Amount, before 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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
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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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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
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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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(2))
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