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Pension and Other Postretirement Benefits - Schedule of Fair Value of Pension Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	$ 4,646	$ 10,926
Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,768	4,400
Plan Assets Measured At Net Asset Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	2,878	6,526
Other Funds [Member] | Plan Assets Measured At Net Asset Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	178	367
Hedge Funds, Equity [Member] | Plan Assets Measured At Net Asset Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	406	932
Private Equity Funds [Member] | Plan Assets Measured At Net Asset Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	165	466
Real Estate [Member] | Plan Assets Measured At Net Asset Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	185	578
Discretionary and Systematic Macro Hedge Funds [Member] | Plan Assets Measured At Net Asset Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	784	1,671
Equity Securities [Member] | Equity Funds [Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	393	826
Equity Securities [Member] | Equity Funds [Member] | Plan Assets Measured At Net Asset Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	431	1,099
Fixed Income Securities [Member] | Plan Assets Measured At Net Asset Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	729	1,413
Fixed Income Securities [Member] | Intermediate and Long Duration Government Credit [Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	118	2,983
Fixed Income Securities [Member] | Other Funds [Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,111	307
Equity And Fixed Income[ Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,229	3,290
Other Investments [Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	146	284
Other Investments [Member] | Other Funds [Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	65	126
Other Investments [Member] | Real Estate Funds [Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	81	158
Level 1 [Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,622	3,606
Level 1 [Member] | Equity Securities [Member] | Equity Funds [Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	393	826
Level 1 [Member] | Fixed Income Securities [Member] | Intermediate and Long Duration Government Credit [Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	23	2,496
Level 1 [Member] | Fixed Income Securities [Member] | Other Funds [Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,060
Level 1 [Member] | Equity And Fixed Income[ Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,083	2,496
Level 1 [Member] | Other Investments [Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	146	284
Level 1 [Member] | Other Investments [Member] | Other Funds [Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	65	126
Level 1 [Member] | Other Investments [Member] | Real Estate Funds [Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	81	158
Level 2 [Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	146	794
Level 2 [Member] | Fixed Income Securities [Member] | Intermediate and Long Duration Government Credit [Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	95	487
Level 2 [Member] | Fixed Income Securities [Member] | Other Funds [Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	51	307
Level 2 [Member] | Equity And Fixed Income[ Member] | Pension Plan Assets Leveled [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	$ 146	$ 794

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Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 30
-Section 35
-Paragraph 50
-URI http://asc.fasb.org/extlink&oid=64937038&loc=d3e12355-114930

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Plan Assets
-URI http://asc.fasb.org/extlink&oid=6749337

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Plan Assets
-URI http://asc.fasb.org/extlink&oid=6520858

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