Income Taxes - Schedule of Components of Net Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Deferred tax assets, Depreciation	$ 187	$ 264
Deferred tax assets, Employee benefits	1,240	286
Deferred tax assets, Loss provisions	313	302
Deferred tax assets, Deferred income/expense	28	48
Deferred tax assets, Tax loss carryforwards	1,064	992
Deferred tax assets, Tax credit carryforwards	23	15
Deferred tax assets, Derivatives and hedging activities	0	0
Deferred tax assets, Other	233	420
Deferred tax assets, Gross	3,088	2,327
Deferred tax assets, Valuation allowance	(1,755)	(712)	$ (486)	$ (517)
Deferred tax assets, net	1,333	1,615
Deferred tax liabilities, Depreciation	499	529
Deferred tax liabilities, Employee benefits		39
Deferred tax liabilities, Loss provisions		7
Deferred tax liabilities, Deferred income/expense	136	312
Deferred tax liabilities, Tax loss carryforwards	0	0
Deferred tax liabilities, Tax credit carryforwards	0	0
Deferred tax liabilities, Derivatives and hedging activities	124	216
Deferred tax liabilities, Other	125	412
Deferred tax liabilities, Gross	884	1,515
Deferred tax liabilities, Valuation allowance	0	0
Deferred tax liabilities, Net	$ 884	$ 1,515