Pension and Other Postretirement Benefits - Assumptions and 401(k) Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Benefit payments including expected future services [Abstract]
Expense for matching contributions in the 401K plan	$ 106	$ 98	$ 91
Pension Benefits [Member]
Assumptions for pension and other postretirement benefits [Abstract]
Discount rate: Benefit obligation	4.17%	3.75%
Discount rate: Benefit cost	3.75%	4.50%
Expected long-term return on plan assets: Benefit cost	7.25%	7.25%
Recognition Period For Market Fluctuation Impacts On Pension Cost	5 years
Defined Benefit Plan Market Related Value Of Pension Plan Assets	$ 3,900
Defined Benefit Plan Market Value Of Pension Plan Assets	4,000
Benefit payments including expected future services [Abstract]
Paid in 2016	363
Paid in 2017	322
Paid in 2018	323
Paid in 2019	329
Paid in 2020	322
Paid in 2021-2025	$ 1,604
Other Postretirement Benefits [Member]
Assumptions for pension and other postretirement benefits [Abstract]
Discount rate: Benefit obligation	3.89%	3.50%
Discount rate: Benefit cost	3.50%	4.00%
Expected long-term return on plan assets: Benefit cost	5.00%	5.00%
Benefit payments including expected future services [Abstract]
Paid in 2016	$ 29
Paid in 2017	28
Paid in 2018	27
Paid in 2019	26
Paid in 2020	25
Paid in 2021-2025	$ 104