Income Tax (Reconciliation of Unrecognized Tax Benefits) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Unrecognized Tax Benefits
Balance at January 1,		$ 1,323	$ 779	$ 774
Additions for tax positions of prior years (1)		26	579	74
Reductions for tax positions of prior years	$ (792)	(124)	(24)	(88)
Additions for tax positions of current year		28	28	23
Reductions for tax positions of current year		0	(1)	0
Settlements with tax authorities		(49)	(38)	(4)
Balance at December 31,		1,204	1,323	779
Unrecognized tax benefits that, if recognized would impact the effective rate		$ 1,170	$ 1,268	$ 690