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]
Tax loss carryforwards	$ 1,185	$ 1,064
Deferred tax assets, Employee benefits	949	1,240
Deferred tax assets, Derivatives and hedging activities	287
Deferred tax assets, Loss provisions	246	313
Deferred tax assets, Tax credit carryforwards	193	23
Deferred tax assets, Depreciation	141	187
Deferred tax assets, Deferred income/expense	11	28
Deferred tax assets, Other	100	233
Deferred tax assets, Gross	3,112	3,088
Deferred tax assets, Valuation allowance	(1,927)	(1,755)	$ (712)	$ (486)
Deferred tax assets, net	1,185	1,333
Deferred tax liabilities, Tax loss carryforwards	0	0
Deferred tax liabilities, Employee benefits	0	0
Deferred tax liabilities, Derivatives and hedging activities	70	124
Deferred tax liabilities, Loss provisions	0	0
Deferred tax liabilities, Tax credit carryforwards	0	0
Deferred tax liabilities, Depreciation	432	499
Deferred tax liabilities, Deferred income/expense	109	136
Deferred tax liabilities, Other	57	125
Deferred tax liabilities, Gross	668	884
Deferred tax liabilities, Valuation allowance	0	0
Deferred tax liabilities, Net	$ 668	$ 884