Operating Segments - Geographic Data (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
Geographic Data
Revenues	$ 5,963	$ 5,833	$ 3,225	$ 8,269	$ 9,154	$ 7,886	$ 8,253	$ 8,375	$ 23,290	$ 33,669	$ 34,055
Long-lived assets	20,779				21,500				20,779	21,500
United States
Geographic Data
Revenues									17,942	27,162	28,362
Long-lived assets	16,887				17,565				16,887	17,565
Canada
Geographic Data
Revenues									5,348	6,507	$ 5,693
Long-lived assets	$ 3,892				$ 3,935				$ 3,892	$ 3,935