Pension and Other Postretirement Benefits - Schedule of Fair Value of Pension Plan Assets (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	$ 10,421	$ 11,043
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	3,852	3,635
Equity Securities [Member] | Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	2,588	2,329
Equity Securities [Member] | Hedge Funds, Equity Long (Short) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	744	756
Equity Securities [Member] | Private Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	520	550
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	4,271	5,580
Fixed Income Securities [Member] | Intermediate and Long Duration Government Credit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	3,802	5,073
Fixed Income Securities [Member] | Other Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	469	507
Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	2,298	1,828
Other Investments [Member] | Other Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	375	459
Other Investments [Member] | Real Estate Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	636	573
Other Investments [Member] | Discretionary and Systematic Macro Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,287
Other Investments [Member] | Discretionary Macro Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets		796
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	3,602	2,510
Level 1 [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,084	1,016
Level 1 [Member] | Equity Securities [Member] | Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,084	1,016
Level 1 [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	2,251	1,169
Level 1 [Member] | Fixed Income Securities [Member] | Intermediate and Long Duration Government Credit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	2,251	1,169
Level 1 [Member] | Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	267	325
Level 1 [Member] | Other Investments [Member] | Other Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	143	212
Level 1 [Member] | Other Investments [Member] | Real Estate Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	124	113
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	3,398	5,414
Level 2 [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,360	1,196
Level 2 [Member] | Equity Securities [Member] | Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,360	1,196
Level 2 [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	2,020	4,196
Level 2 [Member] | Fixed Income Securities [Member] | Intermediate and Long Duration Government Credit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,551	3,689
Level 2 [Member] | Fixed Income Securities [Member] | Other Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	469	507
Level 2 [Member] | Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	18	22
Level 2 [Member] | Other Investments [Member] | Real Estate Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	18	22
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	3,421	3,119
Level 3 [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,408	1,423
Level 3 [Member] | Equity Securities [Member] | Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	144	117
Level 3 [Member] | Equity Securities [Member] | Hedge Funds, Equity Long (Short) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	744	756
Level 3 [Member] | Equity Securities [Member] | Private Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	520	550
Level 3 [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets		215
Level 3 [Member] | Fixed Income Securities [Member] | Intermediate and Long Duration Government Credit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets		215
Level 3 [Member] | Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	2,013	1,481
Level 3 [Member] | Other Investments [Member] | Other Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	232	247
Level 3 [Member] | Other Investments [Member] | Real Estate Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	494	438
Level 3 [Member] | Other Investments [Member] | Discretionary and Systematic Macro Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,287
Level 3 [Member] | Other Investments [Member] | Discretionary Macro Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets		$ 796