Income taxes - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Provision for income taxes	$ 187	$ 258
Estimated annual effective tax rate	20.00%	30.00%
Statutory corporate tax rate	21.00%
Additional tax expense			$ 773
Non-U.S. earnings, Increase (decrease) in income tax expense	$ 669		714
Remeasurement of deferred tax balance			$ 59