Retirement Plans (Narrative) (Details) - USD ($) $ in Millions		12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Mar. 31, 2020
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Capitalization of pension and OPEB expense related to engineering and plant construction		$ 25	$ 24	$ 26
Contribution of Property				37
Contributions to plans	$ 27
Decrease in plan assets		$ (223)
Percentage of increase in pension plan		8.00%
Accumulated benefit obligation	3,640	$ 3,640	3,646
Remaining required plan contributions for fiscal year	120	120			$ 147
401(k) savings plan employer contributions		39	44	$ 45
Plan assets	2,507	2,507	2,730
Benefit payments		538
Special termination benefits			$ 44
Required plan contributions for plan year	$ 173	$ 173
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate - used at year end to value obligation (in hundredths)	2.60%	2.60%	3.40%	4.30%
Expected long-term rate of return on plan assets (in hundredths)		7.50%	7.50%	7.50%
Expected Long Term Rate Of Return On Plan Assets In Future Year In Hundredths		7.50%
Defined Benefit Plan, Settlements, Benefit Obligation		$ 465	$ 235
Defined Benefit Plan, Settlements Benefit obligation threshold		211
Pension settlement costs		159	57	$ 41
Remeasurement charges of other comprehensive income (loss)		16	145
Contribution of Property				37
Contributions to plans		$ 64	166
Employer cash contributions				113
Defined Benefit Plan, Funded Percentage	68.00%	68.00%
Actuarial (gain) loss		$ 506	603
Plan contributions required for fiscal year			166	150	$ 180
Plan assets	$ 2,507	2,507	2,730	$ 2,348
Benefit payments		73	65
Investment returns		(321)	(516)
Special termination benefits			38
Postretirement Benefits Other Than Pensions (OPEB) [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Remeasurement charges of other comprehensive income (loss)		119	(4)
Contributions to plans		52	40
Increase (decrease) in accumulated benefit obligation for plan amendment			149
Actuarial (gain) loss		$ 100	129
Annual rate of increase in the per-capita cost of covered medical benefits (in hundredths)	7.00%	7.00%
Annual rate of increase in the per-capita cost of covered medical benefits in 2029 (in hundredths)	5.00%	5.00%
Benefit payments		$ 61	47
Special termination benefits			$ 6
Postretirement Benefits Other Than Pensions (OPEB) [Member] | Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate - used at year end to value obligation (in hundredths)	2.60%	2.60%	3.40%	4.30%
Postretirement Benefits Other Than Pensions (OPEB) [Member] | Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate - used at year end to value obligation (in hundredths)	2.80%	2.80%	3.50%	4.40%
Fixed Income Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target asset allocation	40.00%	40.00%
Equity Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target asset allocation	60.00%	60.00%
Management Expenses And Other Expenses [Member] | Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Investments in pension plan assets	$ 321	$ 321
Northwest Operations [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Settlements, Benefit Obligation		159
Remeasurement charges of other comprehensive income (loss)		506
Northwest Operations [Member] | Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Investment returns		$ (70)