Leases - Summary of Information Related to Leases (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Leases [Abstract]
Operating leases, assets	$ 534
Finance leases, assets	186
Operating leases, current liabilities	245
Finance leases, current liabilities	21
Operating leases, long-term liabilities	306
Finance leases, long-term liabilities	229
Operating leases, total lease liabilities	551
Finance leases, total lease liabilities	$ 250
Weighted-average remaining lease term, operating leases	5 years
Weighted-average remaining lease term, finance leases	16 years
Weighted-average discount rate, operating leases	4.30%
Weighted-average discount rate, finance leases	15.40%
Operating leases, additions to ROU assets and lease liabilities	$ 60
Finance leases, additions to ROU assets and lease liabilities	$ 6