Employee Benefits - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation for the pension plans	$ 2,923	$ 2,381
Estimated aggregate net actuarial loss and prior service cost to be amortized next year	65
Effect of one percent increase in health care cost trend rate on the accumulated other postretirement benefit obligation	92
Effect of one percent decrease in health care cost trend rate on the accumulated other postretirement benefit obligation	73
Effect of one percent increase in health care cost trend rate on net other postretirement benefit cost	5
Effect of one percent decrease in health care cost trend rate on net other postretirement benefit cost	4
Postretirement benefit, description	The health care cost trend rate assumption has a significant effect on the amount of the accumulated other postretirement benefit obligation and the net other postretirement benefit cost reported. To illustrate, a one percent increase in the trend rate for each year would increase the accumulated other postretirement benefit obligation at December 31, 2014 by approximately $92 million and the aggregate of the service and interest cost components of net other postretirement benefit cost for the year ended December 31, 2014 by approximately $5 million. A one percent decrease in the trend rate for each year would decrease the accumulated other postretirement benefit obligation at December 31, 2014 by approximately $73 million and the aggregate of the service and interest cost components of net other postretirement benefit cost for the year ended December 31, 2014 by approximately $4 million.
Employer elective matching contributions related to employee's eligible pre-tax contributions	Equal to 100%
Employer contributions for Capital Accumulation Plan	30	29	26
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
The portion of employer elective matching contribution related to employee's eligible pre-tax contributions	4.00%
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Minimum range of portfolio allocation to equity securities	55.00%
Maximum range of portfolio allocation to equity securities	65.00%
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets invested in pooled funds	1,556	1,425
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets invested in pooled funds	$ 143	$ 130