Fair Value Measurements (Details 4) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
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	$ 261	$ 269	$ 627	$ 1,089
Realized Gains	3	20	31	70
Realized Losses	(7)	(11)	(34)	(55)
Interest and Dividend Income	$ 11	$ 9	$ 31	$ 28