Investment securities - Net realized and unrealized investment gains (losses) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Net realized and unrealized investment gains (losses)
Gross realized gains	$ 26.9	$ 12.0	$ 41.0	$ 20.4
Gross realized (losses)	(11.3)	(4.2)	(16.4)	(16.3)
Net realized gains (losses) on investments	15.6	7.8	24.6	4.1
Net unrealized investment gains (losses)	15.5	24.7	89.5	40.7
Net realized and unrealized gains (losses) on investments	31.1	32.5	114.1	44.7
Realized gains due to foreign currency	9.7	7.1	20.5	6.0
Unrealized (losses) gains due to foreign currency	(7.2)	29.7	17.8	48.6
Total unrealized investment gains (losses) - Level 3 investments		5.2	8.8	5.6
Fixed maturity investments
Net realized and unrealized investment gains (losses)
Net realized gains (losses) on investments	8.1	3.6	14.9	0.8
Net unrealized investment gains (losses)	11.0	17.8	40.7	20.8
Total unrealized investment gains (losses) - Level 3 investments		(2.1)		(2.1)
Equity securities
Net realized and unrealized investment gains (losses)
Net realized gains (losses) on investments	(1.2)	1.7	(1.8)	0.2
Net unrealized investment gains (losses)	13.3	(5.2)	38.4	1.2
Short-term investments
Net realized and unrealized investment gains (losses)
Net realized gains (losses) on investments	3.1		3.2
Net unrealized investment gains (losses)	(0.9)		1.8
Derivative instruments
Net realized and unrealized investment gains (losses)
Net realized gains (losses) on investments	(0.2)		(0.8)
Net unrealized investment gains (losses)	(0.1)		(0.7)
Other long-term investments
Net realized and unrealized investment gains (losses)
Net realized gains (losses) on investments	5.8	2.5	9.1	3.1
Net unrealized investment gains (losses)	$ (7.8)	12.1	9.3	18.7
Total unrealized investment gains (losses) - Level 3 investments		$ 7.3	$ 8.8	$ 7.7

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 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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