Investment securities - Fixed maturity investments (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Fixed maturity investments
Cost or amortized cost	$ 1,705.0	$ 1,656.6
Gross unrealized gains	37.5	18.7
Gross unrealized losses	(42.0)	(8.2)
Net foreign currency gains (losses)	31.6	13.9
Fair value	$ 1,732.1	1,681.0
Weighted average duration of fixed income portfolio including short-term investments	1 year 6 months
Weighted average duration of fixed income portfolio excluding short-term investments	2 years 3 months
Cost or amortized cost
Due in one year or less	$ 106.2	85.0
Due after one year through five years	429.1	479.1
Due after five years through ten years	45.9	46.3
Due after ten years	25.2	25.1
Fair value
Due in one year or less	112.9	88.4
Due after one year through five years	446.3	490.3
Due after five years through ten years	47.0	46.0
Due after ten years	28.8	24.6
Fixed maturity investments
Fixed maturity investments
Fair value	1,732.1	1,681.0
Fixed maturity investments \| AAA
Fixed maturity investments
Fair value	588.4	559.8
Fixed maturity investments \| AA
Fixed maturity investments
Fair value	786.8	724.3
Fixed maturity investments \| A
Fixed maturity investments
Fair value	208.5	219.0
Fixed maturity investments \| BBB
Fixed maturity investments
Fair value	88.0	95.8
Fixed maturity investments \| Other
Fixed maturity investments
Fair value	60.4	82.1
Corporate debt securities
Fixed maturity investments
Cost or amortized cost	424.5	458.6
Gross unrealized gains	2.8	5.2
Gross unrealized losses	(5.0)	(1.2)
Net foreign currency gains (losses)	22.0	11.5
Fair value	444.3	474.1
Asset-backed securities
Fixed maturity investments
Cost or amortized cost	551.9	489.4
Gross unrealized gains	1.9	1.4
Gross unrealized losses	(30.1)	(3.9)
Net foreign currency gains (losses)	0.7	(0.1)
Fair value	524.4	486.8
Residential mortgage-backed securities
Fixed maturity investments
Cost or amortized cost	443.5	426.2
Gross unrealized gains	23.1	10.5
Gross unrealized losses	(1.2)	(1.4)
Net foreign currency gains (losses)	6.5	3.6
Fair value	471.9	438.9
U.S. Government and government agency
Fixed maturity investments
Cost or amortized cost	119.9	111.5
Gross unrealized gains	6.8	0.7
Gross unrealized losses	(0.1)	(0.4)
Net foreign currency gains (losses)	1.6	(1.3)
Fair value	128.2	110.5
Commercial mortgage-backed securities
Fixed maturity investments
Cost or amortized cost	97.4	88.5
Gross unrealized gains	1.6	0.9
Gross unrealized losses	(5.0)	(0.6)
Net foreign currency gains (losses)	0.3	0.2
Fair value	94.3	89.0
Non-U.S. government and government agency
Fixed maturity investments
Cost or amortized cost	60.4	63.7
Gross unrealized gains	0.4	0.0
Gross unrealized losses	(0.4)	(0.7)
Net foreign currency gains (losses)	0.4	0.0
Fair value	60.8	63.0
Preferred stocks
Fixed maturity investments
Cost or amortized cost	5.8	17.0
Gross unrealized gains	0.9	0.0
Gross unrealized losses	(0.2)	0.0
Net foreign currency gains (losses)	0.0	0.0
Fair value	6.5	17.0
Cost or amortized cost
Cost or amortized cost without single maturity date	5.8	17.0
Fair value
Fair value without single maturity date	6.5	17.0
U.S. States, municipalities and political subdivision
Fixed maturity investments
Cost or amortized cost	1.6	1.7
Gross unrealized gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Net foreign currency gains (losses)	0.1	0.0
Fair value	1.7	1.7
Mortgage-backed and asset-backed securities
Cost or amortized cost
Cost or amortized cost without single maturity date	1,092.8	1,004.1
Fair value
Fair value without single maturity date	1,090.6	1,014.7
Subprime \| Fixed maturity investments
Fixed maturity investments
Fair value	37.8	43.3
Subprime \| Fixed maturity investments \| AAA
Fixed maturity investments
Fair value	18.3	21.7
Subprime \| Fixed maturity investments \| AA
Fixed maturity investments
Fair value	17.2	18.4
Subprime \| Fixed maturity investments \| A
Fixed maturity investments
Fair value	2.2	3.1
Subprime \| Fixed maturity investments \| Other
Fixed maturity investments
Fair value	$ 0.1	$ 0.1

Amount of foreign currency gain (loss) on investment in debt security measured at amortized cost (held-to-maturity).

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Weighted average duration of fixed income portfolio Excluding short-term investments in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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