Investment securities - Net Realized and Unrealized Investment (Losses) Gains (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Realized and Unrealized Investment (Losses) Gains
Gross realized gains	$ 42.3	$ 24.4	$ 310.8
Gross realized (losses)	(40.0)	(51.6)	(22.5)
Net realized gains (losses) on investments(1)(2)	2.3	(27.2)	288.3
Net unrealized investment gains (losses)	(23.2)	(10.5)	(238.2)
Net realized and unrealized investment (losses) gains on investments	(20.9)	(37.7)	50.1
Realized gains (losses) due to foreign currency	17.3	(19.1)	50.0
Unrealized gains (losses) due to foreign currency	35.7	(51.7)	(7.7)
Total unrealized investment (losses) gains - Level 3 investments	(2.1)	(0.7)	1.1
Symetra
Net Realized and Unrealized Investment (Losses) Gains
Net realized gains (losses) on investments(1)(2)			222.5
Fixed maturity investments
Net Realized and Unrealized Investment (Losses) Gains
Net realized gains (losses) on investments(1)(2)	3.6	(19.1)	57.4
Net unrealized investment gains (losses)	14.1	(41.3)	(25.0)
Total unrealized investment (losses) gains - Level 3 investments	(6.1)	(0.2)	(0.1)
Equity securities
Net Realized and Unrealized Investment (Losses) Gains
Net realized gains (losses) on investments(1)(2)	(6.4)	0.1	231.9
Net unrealized investment gains (losses)	(51.1)	25.2	(214.5)
Total unrealized investment (losses) gains - Level 3 investments		0.1
Other long-term investments
Net Realized and Unrealized Investment (Losses) Gains
Net realized gains (losses) on investments(1)(2)	5.1	(8.2)	(1.0)
Net unrealized investment gains (losses)	13.8	5.6	1.3
Total unrealized investment (losses) gains - Level 3 investments	$ 4.0	$ (0.6)	$ 1.2

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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