Income Taxes - Schedule of Components of Net Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Deferred tax assets, depreciation	$ 38	$ 31
Deferred tax assets, employee benefits	836	936
Deferred tax assets, loss provisions	94	134
Deferred tax assets, deferred income	22	19
Deferred tax assets, tax loss carryforwards	3,159	3,305
Deferred tax assets, tax credit carryforwards	579	638
Deferred tax assets, other	94	24
Deferred tax assets, gross	4,822	5,087
Valuation allowance	(2,486)	(2,584)	$ (1,940)	$ (1,291)
Deferred tax assets, net	2,336	2,503
Deferred tax liabilities, depreciation	694	693
Deferred tax liabilities, employee benefits	27	23
Deferred tax liabilities, loss provisions	0	14
Deferred tax liabilities, deferred expense	1,102	1,144
Deferred tax liabilities, tax loss carryforwards	0	0
Deferred tax liabilities, tax credit carryforwards	0	0
Deferred tax liabilities, other	20	33
Deferred tax liabilities, gross	1,843	1,907
Deferred tax liabilities, valuation allowance	0	0
Deferred tax liabilities, net	$ 1,843	$ 1,907