Commitments and Contingencies Long Term Supply and Capacity Purchase Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Long-term Purchase Commitment [Line Items]
Long term purchase committments costs incurred	$ 222,500	$ 209,300	$ 228,400
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
Purchase obligation due in next twelve months	186,500
Purchase obligation due in second year	146,500
Purchase obligation due in third year	150,400
Purchase obligation due in fourth year	150,300
Purchase obligation due in fifth year	146,000
Purchase obligation due thereafter	$ 1,100,000
Maximum [Member]
Long-term Purchase Commitment [Line Items]
Long term purchase commitments term	24 years
Hydroelectric License Commitments [Member]
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
Hydro MOU commitment	$ 17,400
Recoverable Amounts [Member] | Qualifying Facility Contracts [Member]
Long-term Purchase Commitment [Line Items]
Recorded Unconditional Purchase Obligation	$ 508,195