Employee Benefit Plans (Narrative) (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Percentage of non-matching employer's contribution to eligible participants (as a percent)	1.00%
Employer's Common stock held under 401 (k) plan (in shares)	12
Market value of employer's Common stock held under 401 (k) plan	$ 426
Cash dividend received on employer's Common stock held under 401 (k) plan	13
Accumulated other comprehensive income, unrecognized prior service cost (credit)	4
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated other comprehensive income, unrecognized actuarial losses	851
Prior service cost (credit), expected to be recognized in net periodic benefit cost (credit) in next fiscal year	2
Actuarial loss, expected to be recognized in net periodic benefit cost in next fiscal year	56
Defined Benefit Plan, Plans with Benefit Obligations in Excess of Plan Assets, Aggregate Projected Benefit obligation	2,500	$ 2,900
Defined Benefit Plan, Plans with Benefit Obligations in Excess of Plan Assets, Aggregate Benefit Obligation	2,200	2,600
Defined Benefit Plan, Plans with Benefit Obligations in Excess of Plan Assets, Aggregate Fair Value of Plan Assets	1,500	1,700
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets, Aggregate Projected Benefit Obligation	2,500	2,800
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets, Aggregate Accumulated Benefit Obligation	2,200	2,500
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets, Aggregate Fair Value of Plan Assets	1,500	1,700
Accumulated benefit obligation	2,600	$ 3,000
Contributions and expected future benefit payments	$ 31
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Annual index on credits used to fund Health Care Reimbursement accounts	2.00%
Accumulated other comprehensive income, unrecognized prior service cost (credit)	$ (68)
Accumulated other comprehensive income, unrecognized actuarial losses	41
Prior service cost (credit), expected to be recognized in net periodic benefit cost (credit) in next fiscal year	(17)
Actuarial loss, expected to be recognized in net periodic benefit cost in next fiscal year	$ 3
Annual rate of increase in per capita cost of covered health care benefits assumed for transition period (as a percent)	7.25%
Annual rate for per capita cost of covered health care benefits assumed for 2025 and thereafter (as a percent)	4.50%
Year ultimate rate assumed for per capita cost of health care benefits is to be reached	2025
Contributions and expected future benefit payments	$ 13