Pension and Other Postretirement Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	$ 2,975	$ 2,562	$ 2,397
Pension Benefits | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	1,135	1,181
Pension Benefits | Fixed-Income Securities
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	1,109	813
Pension Benefits | Alternatives
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	527	359
Pension Benefits | Insurance Contract
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	18	21
Pension Benefits | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	186	188
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	56	57	57
Other Postretirement Benefits | Deposit Administration Fund
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	56	57
Level 1 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	291	388
Level 1 | Pension Benefits | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	254	388
Level 1 | Pension Benefits | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	37
Level 2 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	2,397	1,919
Level 2 | Pension Benefits | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	881	793
Level 2 | Pension Benefits | Fixed-Income Securities
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	1,100	813
Level 2 | Pension Benefits | Alternatives
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	267	148
Level 2 | Pension Benefits | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	149	165
Level 3
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	343	312	$ 293
Level 3 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	287	255
Level 3 | Pension Benefits | Fixed-Income Securities
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	9
Level 3 | Pension Benefits | Alternatives
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	260	211
Level 3 | Pension Benefits | Insurance Contract
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	18	21
Level 3 | Pension Benefits | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets		23
Level 3 | Other Postretirement Benefits | Deposit Administration Fund
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement plan assets	$ 56	$ 57