Investment securities - Net realized and unrealized investment gains (losses) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Net realized and unrealized investment gains (losses)
Gross realized gains	$ 42.0	$ 14.1
Gross realized (losses)	(21.7)	(5.1)
Net realized gains on investments	20.3	9.0
Net unrealized investment gains (losses)	(43.8)	74.0
Net realized and unrealized gains (losses) on investments	(23.5)	83.0
Realized gains due to foreign currency	11.1	10.9
Unrealized (losses) gains due to foreign currency	52.7	25.0
Total unrealized investment (losses) gains – Level 3 investments	(0.6)	8.8
Fixed maturity investments
Net realized and unrealized investment gains (losses)
Net realized gains on investments	7.8	6.8
Net unrealized investment gains (losses)	7.9	29.7
Total unrealized investment (losses) gains – Level 3 investments	0.3	0.0
Equity securities
Net realized and unrealized investment gains (losses)
Net realized gains on investments	0.5	(0.6)
Net unrealized investment gains (losses)	(69.1)	25.1
Total unrealized investment (losses) gains – Level 3 investments	0.0	0.0
Short-term investments
Net realized and unrealized investment gains (losses)
Net realized gains on investments	2.8	0.1
Net unrealized investment gains (losses)	15.7	2.7
Derivative instruments
Net realized and unrealized investment gains (losses)
Net realized gains on investments	8.3	(0.6)
Net unrealized investment gains (losses)	0.1	(0.6)
Other long-term investments
Net realized and unrealized investment gains (losses)
Net realized gains on investments	0.9	3.3
Net unrealized investment gains (losses)	1.6	17.1
Total unrealized investment (losses) gains – Level 3 investments	$ (0.9)	$ 8.8

Amount of unrealized gain (loss) on investment in marketable security without readily determinable fair value.

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Amount of realized gains on investments during the period, not including gains on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

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Amount of realized losses on investments during the period, not including losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

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Amount before tax of foreign currency transaction realized gain (loss) recognized in the income statement.

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Amount before tax of foreign currency transaction unrealized gain (loss) recognized in the income statement.

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Amount of realized and unrealized gain (loss) on investment.

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The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of investments, not including unrealized gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity, held at each balance sheet date and included in earnings for the period.

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