Goodwill and other intangible assets - Intangible assets - Narrative (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible asset amortization expense	$ 1,469	$ 1,490	$ 1,106
Estimated amortization expense - 2018	1,400
Estimated amortization expense - 2019	1,270
Estimated amortization expense - 2020	1,175
Estimated amortization expense - 2021	1,086
Estimated amortization expense - 2022	1,031
Intangible assets with indefinite lives, excluding goodwill	$ 18,930	$ 18,705