Income Taxes - Schedule of Components of Net Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Deferred tax assets, depreciation	$ 31	$ 15
Deferred tax assets, employee benefits	936	1,382
Deferred tax assets, loss provisions	134	181
Deferred tax assets, deferred income	19	20
Deferred tax assets, tax loss carryforwards	3,305	1,540
Deferred tax assets, tax credit carryforwards	638	652
Deferred tax assets, other	24	164
Deferred tax assets, gross	5,087	3,954
Valuation allowance	(2,584)	(1,940)	$ (1,291)	$ (1,151)
Deferred tax assets, net	2,503	2,014
Deferred tax liabilities, depreciation	693	817
Deferred tax liabilities, employee benefits	23	8
Deferred tax liabilities, loss provisions	14	1
Deferred tax liabilities, deferred expense	1,144	74
Deferred tax liabilities, tax loss carryforwards	0	0
Deferred tax liabilities, tax credit carryforwards	0	0
Deferred tax liabilities, other	33	19
Deferred tax liabilities, gross	1,907	919
Deferred tax liabilities, valuation allowance	0	0
Deferred tax liabilities, net	$ 1,907	$ 919