Income Taxes, Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrecognized Tax Benefits [Roll Forward]
Balance as of beginning of year	$ 936	$ 964	$ 989
Additions based on tax positions related to the current year	33	36	36
Additions for tax positions related to prior years	15	11	83
Reductions for tax positions related to prior years	(42)	(46)	(82)
Reductions for tax positions related to the lapse of applicable statute of limitations	(1)	(3)	(3)
Settlements	0	(237)	(59)
Reclassification of uncertain tax receivable to long-term receivable from IRS	0	211	0
Balance as of end of year	$ 941	$ 936	$ 964