Fair Value Measurements - Proceeds from the Sale of Investments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Proceeds from the sale of investments in available-for-sale securities, realized gains and losses on those sales, and interest and dividend income
Sale proceeds	$ 204	$ 261	$ 506	$ 627
Realized gains	8	3	20	31
Realized Losses	(7)	(7)	(18)	(34)
Interest and Dividend Income	$ 10	$ 11	$ 30	$ 31