Fair Value Measurements Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended					6 Months Ended
	Jun. 30, 2020		Mar. 31, 2020	Jun. 30, 2019		Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ (25,213)		$ (2,096)			$ (2,096)
Balance at beginning of period				$ 7,770			$ 3,035
Included in earnings	(2,284)	[1]		(6,190)	[1]	(26,837)	6,135
Included in regulatory assets/liabilities	(39)			382		284	2,279
Settlements	(731)			(1,045)			(15,627)
Settlements						382
Transferred into Level 3							3,993
Transferred into Level 3	0			0		0
Transferred out of Level 3	0			(965)		0
Transferred out of Level 3							137
Balance at end of period	(28,267)		$ (25,213)	(48)		(28,267)	(48)
Fair Value Measurements, Sensitivity Analysis, Hypothetical Increase or Decrease of Market Prices, Result on Fair Value	(5,600)					(5,600)		$ (2,500)
Rocky Reach
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Change in forward prices			7.60%
Electric Portfolio [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	(26,305)		$ (3,378)			(3,378)
Balance at beginning of period				5,012			1,362
Included in earnings	(2,284)			(6,190)		(26,837)	6,135
Included in regulatory assets/liabilities	0			0		0	0
Settlements	(20)						(12,909)
Settlements				574		1,606
Transferred into Level 3							4,391
Transferred into Level 3	0			0		0
Transferred out of Level 3	0			(1,842)		0	(1,425)
Balance at end of period	(28,609)		(26,305)	(2,446)		(28,609)	(2,446)
Gain (loss) on derivatives	(23,100)			(4,700)		(25,400)	(2,400)
Natural Gas Portfolio [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	1,092		1,282	2,758		1,282	1,673
Included in earnings	0			0		0	0
Included in regulatory assets/liabilities	(39)			382		284	2,279
Settlements	(711)			(1,619)		(1,224)	(2,718)
Transferred into Level 3	0			0		0	(398)
Transferred out of Level 3	0					0
Transferred out of Level 3				877			1,562
Balance at end of period	342		$ 1,092	$ 2,398		342	$ 2,398
Level 2 [Member] | Carrying Value [Member] | Discounted cash flow [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Debt issuance costs	23,100					23,100		24,100
Level 2 [Member] | Carrying Value [Member] | Discounted cash flow [Member] | Subsidiaries [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Debt issuance costs	23,600					23,600		24,400
Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	34,543					34,543		31,308
Derivative Liability	63,941					63,941		26,121
Fair Value, Recurring [Member] | Electric Portfolio [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	20,289					20,289		19,933
Derivative Liability	54,238					54,238		17,504
Fair Value, Recurring [Member] | Natural Gas Portfolio [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	14,254					14,254		11,375
Derivative Liability	9,703					9,703		8,617
Fair Value, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	33,397					33,397		29,134
Derivative Liability	34,528					34,528		21,850
Fair Value, Recurring [Member] | Level 2 [Member] | Electric Portfolio [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	19,763					19,763		19,282
Derivative Liability	25,103					25,103		13,474
Fair Value, Recurring [Member] | Level 2 [Member] | Natural Gas Portfolio [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	13,634					13,634		9,852
Derivative Liability	9,425					9,425		8,376
Fair Value, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	1,146					1,146		2,174
Derivative Liability	29,413					29,413		4,271
Fair Value, Recurring [Member] | Level 3 [Member] | Electric Portfolio [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	526					526		651	[2]
Derivative Liability	29,135					29,135		4,030	[2]
Fair Value, Recurring [Member] | Level 3 [Member] | Natural Gas Portfolio [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	620					620		1,523	[2]
Derivative Liability	$ 278					$ 278		$ 241	[2]

[1] Income Statement locations: Unrealized (gain) loss on derivative instruments, net. Amounts include unrealized gains (losses) on derivatives still held in position as of the reporting date for electric derivatives of $(23.1) million and $(4.7) million for three months ended June 30, 2020 and 2019, respectively. ^2. Income Statement locations: Unrealized (gain) loss on derivative instruments, net. Amounts include unrealized gains (losses) on derivatives still held in position as of the reporting date for electric derivatives of $(25.4) million and $(2.4) million for six months ended June 30, 2020 and 2019, respectively.

[2] The valuation techniques, unobservable inputs and ranges are the same for asset and liability positions.