Defined Benefit Plan, Change in Obligations and Fair Value (Additional Information) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Other Assets	$ 8,949	$ 6,405
Other Liabilities	4,179	2,996
Pension Plan [Member]
Liability, Defined Benefit Plan, Current	78	80
Common Stock Held in Pension Plan	1	2
Other Assets	456	349
Other Liabilities	78	80
Other Postretirement Benefits Plan [Member]
Other Assets	$ 106	$ 99