Debt - Components of Debt (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt
Debt and capital lease obligations	$ 9,491	$ 9,310
Current portion of long-term debt	739	417
Long-term debt, less current portion	8,752	8,893
Revolving Credit Facility [Member]
Debt
Debt and capital lease obligations		426
Credit Facility, aggregate capacity	2,250	$ 2,250
Weighted average interest rate		1.90%
Commercial Paper Program [Member]
Debt
Debt and capital lease obligations	515
Current portion of long-term debt	515
Credit Facility, aggregate capacity	$ 1,500
Weighted average interest rate	1.90%
Canadian Term Loan and Revolving Credit Facility [Member]
Debt
Debt and capital lease obligations	$ 113	$ 239
Weighted average interest rate	2.50%	2.10%
Senior Notes, Aggregate [Member]
Debt
Debt and capital lease obligations	$ 6,184	$ 6,033
Weighted average interest rate	4.30%	4.60%
Tax Exempt Bonds [Member]
Debt
Debt and capital lease obligations	$ 2,352	$ 2,304
Current portion of long-term debt	$ 167
Weighted average interest rate	2.00%	1.80%
Capital Leases and Other [Member]
Debt
Debt and capital lease obligations	$ 327	$ 308
Minimum [Member] \| Senior Notes, Aggregate [Member]
Debt
Interest rate	2.40%
Minimum [Member] \| Tax Exempt Bonds [Member]
Debt
Interest rate	1.20%
Maximum [Member] \| Senior Notes, Aggregate [Member]
Debt
Interest rate	7.75%
Maximum [Member] \| Tax Exempt Bonds [Member]
Debt
Interest rate	5.70%
Maximum [Member] \| Capital Leases and Other [Member]
Debt
Interest rate	12.00%

Sum of the carrying values as of the balance sheet date of all debt, including all short-term borrowings, long-term debt, and capital lease obligations.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.13,16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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- Definition

Amount of short-term debt and current maturity of long-term debt and capital lease obligations due within one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19,20)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Weighted average interest rate of debt outstanding.

+ References

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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Amount of long-term debt and capital lease obligation due after one year or beyond the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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