Leases - Classification of Operating and Finance Leases (Details) - USD ($) $ in Millions	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Operating Leases
Operating lease right-of-use assets	$ 888	$ 899	$ 0
Short-term operating lease liabilities	99	101	0
Long-term operating lease liabilities	858	$ 878	0
Total operating lease liabilities	957
Finance Leases
Property, plant and equipment	7,867		7,778
Less: Accumulated depreciation and amortization	5,730		5,576
Property, plant and equipment, net	2,137		2,202
Current portion of long-term debt	19		19
Long-term debt, net	1,601		$ 1,605
Total finance lease liabilities	4
Operating leases
Operating Leases
Operating lease right-of-use assets	888
Short-term operating lease liabilities	99
Long-term operating lease liabilities	858
Total operating lease liabilities	957
Finance leases
Finance Leases
Property, plant and equipment	7
Less: Accumulated depreciation and amortization	3
Property, plant and equipment, net	4
Current portion of long-term debt	1
Long-term debt, net	3
Total finance lease liabilities	$ 4