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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Investments - Summary of Investment Income (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													$ 200.1	$ 199.1	$ 197.3
Investment expenses													(11.1)	(12.0)	(11.8)
Net investment income	$ 47.5	$ 46.4	$ 47.4	$ 47.7	$ 43.2	$ 46.4	$ 48.0	$ 49.5	$ 46.4	$ 45.0	$ 46.7	$ 47.4	189.0	187.1	185.5
Fixed income securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													133.3	141.3	143.4
Fixed income securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													44.0	31.6	27.8
Short-term Investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													0.4	0.6	1.1
Short-term Investment, Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													0.8	0.2	0.0
Fixed term deposits (included in cash and cash equivalents)
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													6.2	3.4	3.0
Equity securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													0.0	0.0	0.1
Equity securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													13.6	20.4	20.0
Catastrophe bonds — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													$ 1.8	$ 1.6	$ 1.9