Commitments and Contingencies Qualifying Facilities Liability (Details) - USD ($)		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Long Term Purchase Committments Costs Incurred		$ 222,500,000	$ 209,300,000	$ 228,400,000
Beginning QF liability		102,260,000
Ending QF liability		92,937,000	102,260,000
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
Periodic adjustment of the liability for price escalation		14,200,000
Annual reset of liability to actual output and pricing		$ (6,700,000)
Maximum [Member]
Long term purchase commitments term		24 years
Qualifying Facility Contracts [Member]
Beginning QF liability		$ 102,260,000	132,786,000
Unrecovered amount (1)	[1]	(17,257,000)	(39,827,000)
Interest expense		7,934,000	9,301,000
Ending QF liability		92,937,000	$ 102,260,000	$ 132,786,000
Qualifying Facility Contracts [Member] | Minimum [Member]
Price per MWH of energy required to be purchased per QF agreement		63
Qualifying Facility Contracts [Member] | Maximum [Member]
Price per MWH of energy required to be purchased per QF agreement		136
Qualifying Facility Contracts [Member] | Gross Obligation [Member]
Recorded Unconditional Purchase Obligation		630,793,000
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2020		76,533,000
2021		78,356,000
2022		80,226,000
2023		82,320,000
2024		79,726,000
Thereafter		233,632,000
Recorded Unconditional Purchase Obligation		630,793,000
Qualifying Facility Contracts [Member] | Recoverable Amounts [Member]
Recorded Unconditional Purchase Obligation		508,195,000
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2020		59,647,000
2021		60,136,000
2022		60,639,000
2023		61,280,000
2024		60,706,000
Thereafter		205,787,000
Recorded Unconditional Purchase Obligation		508,195,000
Qualifying Facility Contracts [Member] | Net Amount [Member]
Recorded Unconditional Purchase Obligation		122,598,000
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2020		16,886,000
2021		18,220,000
2022		19,587,000
2023		21,040,000
2024		19,020,000
Thereafter		27,845,000
Recorded Unconditional Purchase Obligation		$ 122,598,000

[1] The change in the unrecovered amount includes (i) a lower periodic adjustment of $14.2 million due to price escalation, which was less than previously modeled, and (ii) a lower impact of the annual reset to actual output and pricing resulting in approximately $6.7 million in higher supply costs for these QF contracts due primarily to outages at two facilities in 2018.