Fair Value Measurements, Other Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Financial assets:
Cash and temporary cash investments, at carrying amount	$ 4,658	$ 5,850	$ 4,463	$ 4,816
Financial liabilities:
Debt (excluding capital leases), at carrying amount	8,392	8,310
Fair Value, Inputs, Level 1 [Member]
Financial assets:
Cash and temporary cash investments, at fair value	4,658	5,850
Fair Value, Inputs, Level 2 [Member]
Financial liabilities:
Debt (excluding capital leases), at fair value	$ 9,527	$ 9,795