Leases (Supplemental Cash Flow and Other Information Related to Leases) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Leases [Abstract]
Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases	$ 35
Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from finance leases	2
Cash paid for amounts included in the measurement of lease liabilities: Financing cash flows from finance leases	3
Right-of-use assets obtained in exchange for new finance lease liabilities	1
Right-of-use assets obtained in exchange for new operating lease liabilities	$ 15
Weighted-average remaining lease term: Operating leases	6 years
Weighted-average remaining lease term: Finance leases	20 years
Weighted-average discount rate: Operating leases	9.30%
Weighted-average discount rate: Finance leases	5.70%