Income Taxes - Schedule of Components of Net Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Deferred tax assets, depreciation	$ 21	$ 10
Deferred tax assets, employee benefits	364	368
Deferred tax assets, loss provisions	24	36
Deferred tax assets, deferred income	41	48
Deferred tax assets, interest	3	56
Deferred tax assets, tax loss carryforwards	3,267	2,819
Deferred tax assets, tax credit carryforwards	378	379
Deferred tax assets, other	7	23
Deferred tax assets, gross	4,105	3,739
Valuation allowance	(2,307)	(2,121)	$ (2,357)	$ (2,459)
Deferred tax assets, net	1,798	1,618
Deferred tax liabilities, depreciation	506	480
Deferred tax liabilities, employee benefits	0	6
Deferred tax liabilities, loss provisions	1	0
Deferred tax liabilities, deferred expense	1,033	939
Deferred tax liabilities, interest	0	0
Deferred tax liabilities, tax loss carryforwards	0	0
Deferred tax liabilities, tax credit carryforwards	0	0
Deferred tax liabilities, other	13	0
Deferred tax liabilities, gross	1,553	1,425
Deferred tax liabilities, valuation allowance	0	0
Deferred tax liabilities, net	$ 1,553	$ 1,425