Income taxes - Tax credit carryforward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income taxes
Net operating losses carried forward subject to expiration	$ 1,083
Tax credits carried forward research and development	214
Unrecognized Tax Benefits	94	$ 132	$ 133	$ 119
Interest and penalties accrued	7	15	11
increases/(decreases) in interest and penalties	(7)	$ 4	$ 1
Federal
Income taxes
Net operating losses carried forward subject to expiration	191
Operating losses carried forward	106
Tax credits carried forward research and development	137
Foreign
Income taxes
Net operating losses carried forward subject to expiration	731
U.S.
Income taxes
Net operating losses carried forward not subject to expiration	$ 158
Net operating loss carryforward annual limitation as a percent of taxable income	80.00%
Tax credits carried forward research and development	$ 77
Other Foreign
Income taxes
Net operating losses carried forward not subject to expiration	$ 3