Pension and Other Postretirement Benefits - Schedule of Fair Value of Plan Assets (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 2,603	$ 2,484
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	2,236	2,116	2,039
Pension Benefits [Member] | Equity Securities Domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	376	389
Pension Benefits [Member] | Equity Securities International [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	255	260
Pension Benefits [Member] | Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	1,459	1,309
Pension Benefits [Member] | Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	47	53
Pension Benefits [Member] | Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	54	61
Pension Benefits [Member] | Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	45	44
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	367	368	321
Other Postretirement Benefits [Member] | Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	126	113
Other Postretirement Benefits [Member] | Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	6	22
Other Postretirement Benefits [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	235	233
Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	767	783
Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	427	444
Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] | Pension Benefits [Member] | Equity Securities Domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	128	142
Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] | Pension Benefits [Member] | Equity Securities International [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	254	258
Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] | Pension Benefits [Member] | Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	45	44
Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	340	339
Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] | Other Postretirement Benefits [Member] | Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	126	113
Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] | Other Postretirement Benefits [Member] | Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	6	22
Quoted Prices in Active Markets for Identical Instruments (Level 1) [Member] | Other Postretirement Benefits [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	208	204
Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	1,688	1,541
Significant Other Observable Inputs (Level 2) [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	1,661	1,512
Significant Other Observable Inputs (Level 2) [Member] | Pension Benefits [Member] | Equity Securities Domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	213	213
Significant Other Observable Inputs (Level 2) [Member] | Pension Benefits [Member] | Equity Securities International [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets		1
Significant Other Observable Inputs (Level 2) [Member] | Pension Benefits [Member] | Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	1,448	1,298
Significant Other Observable Inputs (Level 2) [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	27	29
Significant Other Observable Inputs (Level 2) [Member] | Other Postretirement Benefits [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	27	29
Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	148	160	174
Significant Unobservable Inputs (Level 3) [Member] | Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	11	11	13
Significant Unobservable Inputs (Level 3) [Member] | Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	47	53	56
Significant Unobservable Inputs (Level 3) [Member] | Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	54	61	74
Significant Unobservable Inputs (Level 3) [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	36	35	31
Significant Unobservable Inputs (Level 3) [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	148	160
Significant Unobservable Inputs (Level 3) [Member] | Pension Benefits [Member] | Equity Securities Domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	35	34
Significant Unobservable Inputs (Level 3) [Member] | Pension Benefits [Member] | Equity Securities International [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	1	1
Significant Unobservable Inputs (Level 3) [Member] | Pension Benefits [Member] | Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	11	11
Significant Unobservable Inputs (Level 3) [Member] | Pension Benefits [Member] | Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	47	53
Significant Unobservable Inputs (Level 3) [Member] | Pension Benefits [Member] | Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 54	$ 61