Income taxes - Tax credit carryforward (Details) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($)
Income taxes
Net operating losses carried forward subject to expiration	$ 1,665
Federal
Income taxes
Net operating losses carried forward subject to expiration	870
Tax credits carried forward research and development	74
U.S.
Income taxes
Net operating losses carried forward not subject to expiration	$ 264
Net operating loss carryforward annual limitation as a percent of taxable income	80.00%
Tax credits carried forward research and development	$ 6
State
Income taxes
Net operating losses carried forward subject to expiration	795
Tax credits carried forward research and development	$ 2

Net operating loss carryforward annual limitation as a percent of taxable income.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible research tax credit carryforwards.

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