Parent Company (Interest Paid and Income Tax Refunds) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Interest paid	$ 1,743	$ 1,317	$ 1,005
Income taxes refunded	24	111	286
Parent Company
Condensed Financial Statements, Captions [Line Items]
Interest paid	287	317	404
Income taxes refunded	$ 40	$ 183	$ 64