Leases - Information Related to Operating Leases (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Leases [Abstract]
Right-of-use assets	$ 8,272
Lease liabilities	$ 8,562
Weighted average remaining lease term (in years)	8 years 8 months 12 days
Weighted average discount rate	3.74%
Rental expense
Gross rental expense	$ 514
Sublease rental income	(46)
Net rental expense	468
Supplemental cash flow information
Cash paid for amounts included in the measurement of lease liabilities - operating cash flows	389
Supplemental non-cash information
Right-of-use assets obtained in exchange for operating lease obligations	$ 365