Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Unfavorable impact in tax provision from change in tax law	$ 119	$ 0	$ 0
Deferred tax asset amount related to accumulated other comprehensive income	1,000	1,700
Investment of U.S. government accounts receivable in the CCF	692	$ 388
Net operating loss carryforwards	1,000
Capital loss carryforwards	234
Tax credit carryforwards	$ 123
Possible chance of tax position sustainable, percentage	50.00%
Tax positions for which it is reasonably possible that the unrecognized tax benefits will significantly increase or decrease	$ 0