Significant Accounting Policies and New Accounting Pronouncements - Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative adjustment to retained earnings	$ (15,854)	$ (15,916)		$ (15,586)	$ (15,518)	$ (15,549)	$ (15,403)
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative adjustment to retained earnings	$ (1,684)	$ (1,733)		(1,681)	$ (1,599)	$ (1,594)	(1,528)
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative adjustment to retained earnings				1			1
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative adjustment to retained earnings			$ 1	$ 1			$ (11)