Income Taxes - Schedule of Components of Net Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Tax loss carryforwards	$ 1,231	$ 1,185
Deferred tax assets, Employee benefits	683	949
Deferred tax assets, Loss provisions	212	246
Investment basis differences	162	72
Deferred tax assets, Depreciation	91	141
Deferred tax assets, Derivatives and hedging activities	53	287
Deferred tax assets, Tax credit carryforwards	27	193
Deferred tax assets, Deferred income/expense	10	11
Deferred tax assets, Other	87	28
Deferred tax assets, Gross	2,556	3,112
Deferred tax assets, Valuation allowance	(1,684)	(1,927)	$ (1,755)	$ (712)
Deferred tax assets, net	872	1,185
Deferred tax liabilities, Tax loss carryforwards	0	0
Deferred tax liabilities, Employee benefits	0	0
Deferred tax liabilities, Loss provisions	0	0
Investment basis differences	0	0
Deferred tax liabilities, Depreciation	428	432
Deferred tax liabilities, Derivatives and hedging activities	39	70
Deferred tax liabilities, Tax credit carryforwards	0	0
Deferred tax liabilities, Deferred income/expense	103	109
Deferred tax liabilities, Other	1	57
Deferred tax liabilities, Gross	571	668
Deferred tax liabilities, Valuation allowance	0	0
Deferred tax liabilities, Net	$ 571	$ 668