Investments in Unconsolidated Entities - Summarized Financial Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investments in Unconsolidated Entities
Current assets	$ 3,665				$ 4,612				$ 3,665	$ 4,612
Current liabilities	4,253				5,017				4,253	5,017
Noncurrent liabilities	10,506				10,465				10,506	10,465
Revenues	5,963	$ 5,833	$ 3,225	$ 8,269	9,154	$ 7,886	$ 8,253	$ 8,375	23,290	33,669	$ 34,055
Operating income	(65)	$ 253	$ 210	$ (2,773)	331	$ 492	$ 451	$ 714	(2,375)	1,988	2,277
Equity Method Investment, Nonconsolidated Investee
Investments in Unconsolidated Entities
Current assets	580				652				580	652
Noncurrent assets	8,769				7,264				8,769	7,264
Current liabilities	343				298				343	298
Noncurrent liabilities	$ 10				$ 26				10	26
Revenues									1,360	1,469	1,235
Operating income									828	994	824
Net income									$ 826	$ 995	$ 824