Financial Instruments - Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financial assets and financial liabilities measured at fair value on a recurring basis:
Debt securities - current	$ 2,932	$ 608
Derivative assets	694	942
Derivative liabilities	415	415
Potential reduction in net position of total derivative assets	285	255
Recurring
Financial assets and financial liabilities measured at fair value on a recurring basis:
Cash equivalents	8,263	8,586
Equity investments	11	4
Debt securities - current	2,932	608
Debt securities - noncurrent		11
Derivative assets	694	942
Total assets	11,900	10,151
Derivative liabilities	415	415
Potential reduction in net position of total derivative assets	285	255
Potential reduction in net position of total derivative liabilities	285	255
Recurring | Prepaid expenses and other current assets
Financial assets and financial liabilities measured at fair value on a recurring basis:
Derivative assets	312	185
Recurring | Investments and sundry assets
Financial assets and financial liabilities measured at fair value on a recurring basis:
Derivative assets	382	757
Recurring | Other accrued expenses and liabilities
Financial assets and financial liabilities measured at fair value on a recurring basis:
Derivative liabilities	122	377
Recurring | Other liabilities
Financial assets and financial liabilities measured at fair value on a recurring basis:
Derivative liabilities	292	38
Recurring | Time deposits and certificates of deposit
Financial assets and financial liabilities measured at fair value on a recurring basis:
Cash equivalents	8,238	8,066
Recurring | Commercial paper
Financial assets and financial liabilities measured at fair value on a recurring basis:
Cash equivalents		96
Recurring | Money market funds
Financial assets and financial liabilities measured at fair value on a recurring basis:
Cash equivalents	26	26
Recurring | Canadian government securities
Financial assets and financial liabilities measured at fair value on a recurring basis:
Cash equivalents		398
Recurring | Level 1
Financial assets and financial liabilities measured at fair value on a recurring basis:
Cash equivalents	26	26
Equity investments	11	4
Debt securities - noncurrent		4
Derivative assets	9
Total assets	46	33
Derivative liabilities	2
Recurring | Level 1 | Money market funds
Financial assets and financial liabilities measured at fair value on a recurring basis:
Cash equivalents	26	26
Recurring | Level 2
Financial assets and financial liabilities measured at fair value on a recurring basis:
Cash equivalents	8,238	8,560
Debt securities - current	2,932	608
Debt securities - noncurrent		7
Derivative assets	685	942
Total assets	11,854	10,117
Derivative liabilities	413	415
Recurring | Level 2 | Time deposits and certificates of deposit
Financial assets and financial liabilities measured at fair value on a recurring basis:
Cash equivalents	$ 8,238	8,066
Recurring | Level 2 | Commercial paper
Financial assets and financial liabilities measured at fair value on a recurring basis:
Cash equivalents		96
Recurring | Level 2 | Canadian government securities
Financial assets and financial liabilities measured at fair value on a recurring basis:
Cash equivalents		$ 398