Retirement Plans (Narrative) (Details) - USD ($)		3 Months Ended
	Dec. 31, 2020	Mar. 31, 2021	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		$ 6,000,000	$ 7,000,000
Contributions to plans	$ 27,000,000
Remaining required plan contributions for fiscal year	120,000,000
Plan contributions required for fiscal year			147,000,000
Pension Costs [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Settlements Benefit obligation threshold			211,000,000
Pension settlement costs			103,000,000
Contributions to plans		0
Decrease in plan assets		(23,000,000)
Plan contributions required for fiscal year		95,000,000	180,000,000
Plan assets	$ 2,507,000,000	2,484,000,000
Investment returns		(1,000,000)
Settlements			$ 310,000,000
Northwest Operations [Member] | Pension Costs [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Benefit payments		19,000,000
Plan expenses		$ 5,000,000