Debt (Summary Of Long-Term Debt) (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Credit facility	$ 368	$ 0
Unamortized original issue premium (discount), net	(6)	22
Senior notes issuance costs, net	(24)	(31)
Less: current portion	0	0
Total long-term debt	2,738	2,741
5.5% unsecured senior notes due 2022 [Member]
Debt Instrument [Line Items]
Unsecured senior notes	0	600
5.5% unsecured senior notes due 2023 [Member]
Debt Instrument [Line Items]
Unsecured senior notes	0	1,550
4.375% unsecured senior notes due 2025 [Member]
Debt Instrument [Line Items]
Unsecured senior notes	600	600
3.75% unsecured senior notes due 2027 [Member]
Debt Instrument [Line Items]
Unsecured senior notes	1,000	0
4.875% unsecured senior notes due 2047 [Member]
Debt Instrument [Line Items]
Unsecured senior notes	$ 800	$ 0

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
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Amount, after accumulated amortization, of debt discount (premium).

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The carrying value as of the balance sheet date of the noncurrent portion of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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