Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Deferred losses related to NCF Retirement Plan settlement	[1]	$ (60)		$ 0		$ 0
Pension Plan [Member]
Defined Benefit Plan, Service Cost	[2]	6	[3]	5	[3]	5
Defined Benefit Plan, Interest Cost		91	[3]	95	[3]	97
Defined Benefit Plan, Expected Return (Loss) on Plan Assets		(187)		(195)		(186)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		0		0		0
Defined Benefit Plan, Amortization of Gain (Loss)		22		25		25
Deferred losses related to NCF Retirement Plan settlement		60		0		0
Defined Benefit Plan, Other Cost (Credit)		0		0		0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		$ (8)		$ (70)		$ (59)
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate		3.62%		4.18%		4.44%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets		5.90%		6.66%		6.68%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Interest Crediting Rate		3.00%		3.11%		3.00%
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Service Cost		$ 0		$ 0		$ 0
Defined Benefit Plan, Interest Cost		1		1		2
Defined Benefit Plan, Expected Return (Loss) on Plan Assets		(5)		(5)		(5)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		(6)		(6)		(6)
Defined Benefit Plan, Amortization of Gain (Loss)		0		0		0
Deferred losses related to NCF Retirement Plan settlement		0		0		0
Defined Benefit Plan, Other Cost (Credit)		0		9		0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		$ (10)		$ (1)		$ (9)
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate		3.29%		3.70%		3.95%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets		3.10%		3.12%		3.13%

[1] Related to the Company's NCF Retirement Plan settlement in the fourth quarter of 2018. See Note 17, "Employee Benefit Plans," for additional information.

[2] Administrative fees are recognized in service cost for each of the periods presented.

[3] Employer contributions represent the benefits that were paid to nonqualified plan participants. Unfunded nonqualified supplemental pension plans are not funded through plan assets.