Accounting Principles and Practices (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative adjustment amount to decrease retained earnings	$ (3,620)	$ (3,695)	$ (3,260)		$ (3,449)	$ (3,561)	$ (3,913)	$ (4,862)	$ (4,219)
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative adjustment amount to decrease retained earnings	$ (1,423)	$ (1,751)	$ (1,455)		(1,254)	$ (1,436)	$ (1,669)	$ (2,555)	$ (2,093)
Cumulative Effect, Period Of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative adjustment amount to decrease retained earnings					6
Cumulative Effect, Period Of Adoption, Adjustment | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative adjustment amount to decrease retained earnings				$ 6	$ 6