Investment securities - Net Realized and Unrealized Investment gains (losses) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Net Realized and Unrealized Investment Gains (losses)
Gross realized gains	$ 14.1	$ 8.4
Gross realized (losses)	(5.1)	(12.1)
Net realized gains (losses) on investments	9.0	(3.7)
Net unrealized gains on investments	74.0	16.0
Net realized and unrealized investment gains on investments	83.0	12.3
Realized gains (losses) due to foreign currency	10.9	(1.1)
Unrealized gains due to foreign currency	25.0	18.9
Total unrealized investment (losses) gains - Level 3 investments	8.8	0.4
Fixed maturity investments
Net Realized and Unrealized Investment Gains (losses)
Net realized gains (losses) on investments	6.8	(2.8)
Net unrealized gains on investments	29.7	3.0
Short-term investments
Net Realized and Unrealized Investment Gains (losses)
Net realized gains (losses) on investments	0.1
Net unrealized gains on investments	2.7
Equity securities
Net Realized and Unrealized Investment Gains (losses)
Net realized gains (losses) on investments	(0.6)	(1.5)
Net unrealized gains on investments	25.1	6.4
Other long-term investments
Net Realized and Unrealized Investment Gains (losses)
Net realized gains (losses) on investments	3.3	0.6
Net unrealized gains on investments	17.1	6.6
Total unrealized investment (losses) gains - Level 3 investments	8.8	$ 0.4
Derivative instruments
Net Realized and Unrealized Investment Gains (losses)
Net realized gains (losses) on investments	(0.6)
Net unrealized gains on investments	$ (0.6)

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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