Fair Value Hierarchy of Assets Measured at Fair Value on a Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Available for sale securities debt Securities	$ 8,779	$ 8,383
Available for sale securities equity securities	2,956	3,041
Pension Plans, Defined Benefit \| Fair Value, Measurements, Recurring
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Fair value of asset	212	174
Pension Plans, Defined Benefit \| Fair Value, Measurements, Recurring \| Debt Securities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Available for sale securities debt Securities	59	48
Pension Plans, Defined Benefit \| Fair Value, Measurements, Recurring \| Debt Securities \| Corporate Debt Securities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Available for sale securities debt Securities	25	27
Pension Plans, Defined Benefit \| Fair Value, Measurements, Recurring \| Debt Securities \| US States and Political Subdivisions Debt Securities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Available for sale securities debt Securities	34	21
Pension Plans, Defined Benefit \| Fair Value, Measurements, Recurring \| Equity Securities \| Common Stock
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Available for sale securities equity securities	149	122
Pension Plans, Defined Benefit \| Fair Value, Measurements, Recurring \| Equity Securities \| Preferred Stock
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Available for sale securities equity securities	4	4
Pension Plans, Defined Benefit \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Fair value of asset	153	126
Pension Plans, Defined Benefit \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1 \| Equity Securities \| Common Stock
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Available for sale securities equity securities	149	122
Pension Plans, Defined Benefit \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1 \| Equity Securities \| Preferred Stock
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Available for sale securities equity securities	4	4
Pension Plans, Defined Benefit \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Fair value of asset	59	48
Pension Plans, Defined Benefit \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| Debt Securities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Available for sale securities debt Securities	59	48
Pension Plans, Defined Benefit \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| Debt Securities \| Corporate Debt Securities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Available for sale securities debt Securities	25	27
Pension Plans, Defined Benefit \| Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| Debt Securities \| US States and Political Subdivisions Debt Securities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Available for sale securities debt Securities	$ 34	$ 21

For an unclassified balance sheet, total of debt securities categorized neither as held-to-maturity nor trading.

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+ Details

- Definition

For an unclassified balance sheet, this item represents equity securities categorized neither as held-to-maturity nor trading.

+ References

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+ Details

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents investment securities as of the balance sheet date which may include marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References+ Details