Investment securities - Net realized and unrealized investment gains (losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net realized and unrealized investment gains (losses)
Gross realized gains	$ 41.3	$ 22.1	$ 140.7	$ 63.1
Gross realized (losses)	(45.1)	(6.8)	(117.1)	(23.2)
Net realized (losses) gains on investments	(3.8)	15.3	23.6	39.9
Net unrealized investment gains (losses)	31.2	53.9	(3.9)	143.4
Net realized and unrealized gains (losses) on investments	27.4	69.2	19.7	183.3
Realized gains due to foreign currency	(11.2)	14.0	5.4	34.5
Unrealized (losses) gains due to foreign currency	(16.7)	33.9	(29.8)	51.7
Total unrealized investment gains (losses) – Level 3 investments	8.2	(0.3)	3.4	10.1
Fixed maturity investments
Net realized and unrealized investment gains (losses)
Net realized (losses) gains on investments	(0.4)	7.5	17.3	22.4
Net unrealized investment gains (losses)	(1.0)	31.4	2.2	72.1
Total unrealized investment gains (losses) – Level 3 investments	0.0	0.0	0.3	0.0
Equity securities
Net realized and unrealized investment gains (losses)
Net realized (losses) gains on investments	1.0	1.7	(5.0)	(0.1)
Net unrealized investment gains (losses)	18.5	12.0	(33.7)	50.4
Short-term investments
Net realized and unrealized investment gains (losses)
Net realized (losses) gains on investments	(6.2)	6.5	(2.4)	9.7
Net unrealized investment gains (losses)	(3.1)	4.6	(3.2)	6.4
Derivative instruments
Net realized and unrealized investment gains (losses)
Net realized (losses) gains on investments	1.8	(0.7)	10.3	(1.5)
Net unrealized investment gains (losses)	(0.8)	0.9	1.4	0.2
Other Long-term Investments
Net realized and unrealized investment gains (losses)
Net realized (losses) gains on investments	0.0	0.3	3.4	9.4
Net unrealized investment gains (losses)	17.6	5.0	29.4	14.3
Total unrealized investment gains (losses) – Level 3 investments	$ 8.2	$ (0.3)	$ 3.1	$ 10.1