Non-U.S. Activities - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 11,170	$ 10,207	$ 10,360
Assets	238,425	242,698
U.K.
Segment Reporting Information [Line Items]
Revenues	1,050	1,050	938
Non-U.S.
Segment Reporting Information [Line Items]
Revenues	4,734	4,419	$ 4,428
Assets	$ 82,100	$ 79,100