Leases - Summary of the components of net lease cost (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Leases [Abstract]
Operating lease cost	$ 681	$ 2,044
Lease, Cost [Abstract]
Amortization of right-of-use assets	9	27
Interest on lease liabilities	11	32
Total finance lease costs	20	59
Short-term lease costs	5	17
Variable lease costs	148	434
Less: sublease income	13	35
Net lease cost	$ 841	$ 2,519