Significant Accounting Policies and New Accounting Pronouncements - Effect of Change in Retrospective Presentation Related to Net Periodic Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operations and maintenance	$ 533	$ 493	$ 1,060	$ 1,015
Other income / (expense)	$ (20)	(21)	$ (41)	(37)
Improving Presentation of Net Periodic Benefit Costs [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operations and maintenance		493		1,015
Other income / (expense)		(21)		(37)
Improving Presentation of Net Periodic Benefit Costs [Member] | As Previously Reported [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operations and maintenance		522		1,073
Other income / (expense)		8		21
Improving Presentation of Net Periodic Benefit Costs [Member] | Effect of Change Higher/(Lower) [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operations and maintenance		(29)		(58)
Other income / (expense)		$ (29)		$ (58)