Fair Value Measurements - Proceeds from the Sale of Investments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 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	$ 149	$ 175	$ 302	$ 366
Realized Gains	5	9	12	28
Realized Losses	(5)	(11)	(11)	(27)
Interest and Dividend Income	$ 11	$ 10	$ 20	$ 20