Fair Value Measurements - Recurring Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of items measured at fair value on a recurring basis that used significant unobservable inputs (Level 3)
Balance at the beginning of the period	$ 46	$ 40	$ 30
Total gains or losses included in earnings	0	0	0
Total gains or losses included in other comprehensive income	0	0	0
Purchases and issuances	10	9	13
Sales and settlements	(22)	(3)	(3)
Transfers in and/or out of Level 3	0	0	0
Balance at the end of the period	34	46	40
Total gains or losses included in other comprehensive income	$ 0	$ 0	$ 0