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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Investments - Summary of Investment Income (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	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, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													$ 207.1	$ 200.1	$ 199.1
Investment expenses													(8.9)	(11.1)	(12.0)
Net investment income	$ 52.5	$ 48.0	$ 50.4	$ 47.3	$ 47.5	$ 46.4	$ 47.4	$ 47.7	$ 43.2	$ 46.4	$ 48.0	$ 49.5	198.2	189.0	187.1
Fixed income securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													134.1	133.3	141.3
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													1.4	0.4	0.6
Short-term investment, Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													0.4	0.8	0.2
Fixed term deposits (included in cash and cash equivalents)
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													14.2	6.2	3.4
Equity securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													2.1	13.6	20.4
Catastrophe bonds — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													2.8	1.8	1.6
Other investments, at fair value
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													2.5	0.0	0.0
Fixed Income Securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													$ 49.6	$ 44.0	$ 31.6