Employee Benefit Plans (Narrative) (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Increase (Decrease) for Plan Amendment	$ 18
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)	11
Market value of employer's Common stock held under 401 (k) plan	$ 422
Cash dividend received on employer's Common stock held under 401 (k) plan	14
Plan Amendment, Effect on AOCI	164
Plan amendment - effect on funded status	182
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Curtailment	0	$ 0	$ 0
Accumulated other comprehensive income, unrecognized prior service cost (credit)	171
Accumulated other comprehensive income, unrecognized actuarial losses	706
Prior service cost (credit), expected to be recognized in net periodic benefit cost (credit) in next fiscal year	19
Actuarial loss, expected to be recognized in net periodic benefit cost in next fiscal year	60
Defined Benefit Plan, Plans with Benefit Obligations in Excess of Plan Assets, Aggregate Projected Benefit obligation	2,700	2,700
Defined Benefit Plan, Plan with Benefit Obligation in Excess of Plan Assets, Benefit Obligation	2,600	2,300
Defined Benefit Plan, Plan with Benefit Obligation in Excess of Plan Assets, Fair Value of Plan Assets	2,000	1,800
Defined Benefit Plan, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation	2,700	2,600
Defined Benefit Plan, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation	2,500	2,300
Defined Benefit Plan, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Fair Value of Plan Assets	1,900	1,700
Accumulated benefit obligation	3,000	$ 2,700
Contributions and expected future benefit payments	28
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service years		30 years
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Curtailment	0	$ 38	$ 0
Accumulated other comprehensive income, unrecognized prior service cost (credit)	46
Accumulated other comprehensive income, unrecognized actuarial losses	35
Prior service cost (credit), expected to be recognized in net periodic benefit cost (credit) in next fiscal year	15
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)	7.70%
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	2026
Contributions and expected future benefit payments	$ 12