Share-based payment plans - Performance Shares Units (Details) - Performance Share Unit $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Number of common stock	2
Payout percentage	200.00%
Payout percentage in equity	100.00%
Payout percentage in cash	100.00%
Summary of awards
Granted (in shares) \| shares	1,060,253
Cancelled or expired (in shares) \| shares	(96,851)
Outstanding at the end of the period (in shares) \| shares	963,402
Weighted Average Grant Date Fair Value
Granted (per share) \| $ / shares	$ 6.81
Cancelled or expired (per share) \| $ / shares	6.62
Outstanding at the end of the period (per share) \| $ / shares	$ 6.83
Additional disclosures
Total unrecognized compensation expense \| $	$ 7
Weighted average remaining period (in years)	1 year 6 months
Valuation of Share-Based Compensation Awards
Risk-free interest rate, minimum	2.20%
Risk-free interest rate, maximum	2.30%
Volatility, minimum	63.00%
Volatility, maximum	67.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance percentage	0.00%
Valuation of Share-Based Compensation Awards
Dividend yield	0.50%
Expected lives (in years)	2 years 10 months 24 days
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance percentage	200.00%
Valuation of Share-Based Compensation Awards
Dividend yield	0.90%
Expected lives (in years)	3 years

Number of common shares that can be received per performance unit.

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Payout percentage for PSU.

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Payout percentage for PSU in cash.

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Payout percentage for PSU in equity.

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Performance Percentage for Performance Share Units based on ACL and TSR.

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The number of equity-based payment instruments, excluding stock (or unit) options, that was cancelled or expired during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were cancelled or expired.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the maximum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The estimated measure of the minimum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The maximum risk-free interest rate assumption that is used in valuing an option on its own shares.

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The minimum risk-free interest rate assumption that is used in valuing an option on its own shares.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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