Long-Term Debt and Credit Facilities - Interest Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Disclosure [Abstract]
Gross interest expense	$ 1,743	$ 2,093	$ 2,230
Capitalized interest	(75)	(72)	(53)
Total interest expense	$ 1,668	$ 2,021	$ 2,177