Income Taxes Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Effective Tax Rate Reconciliation [Line Items]
Regulatory liabilities	$ 37,954		$ 37,954		$ 113,209
Federal income taxes at statutory rates	$ 4,874	$ 6,479	$ 35,513	$ 20,269
Federal income taxes at statutory rates	21.00%	21.00%	21.00%	21.00%
Tax effect of regulatory treatment of utility plant differences	$ (2,139)	$ (1,981)	$ (4,219)	$ (2,999)
Tax effect of regulatory treatment of utility plant differences	(9.20%)	(6.40%)	(2.50%)	(3.10%)
State income tax expense	$ (8)	$ 60	$ 1,651	$ 1,024
State income tax expense	0.00%	0.20%	1.00%	1.10%
Acquisition costs	$ 112	$ 73	$ (1,712)	$ 119
Acquisition costs	0.50%	0.20%	(1.00%)	0.10%
Non-plant excess deferred turnaround (1)	$ (5,091)	$ (11)	$ (5,601)	$ (11)
Non-plant excess deferred turnaround (1)	(21.90%)	0.00%	(3.30%)	0.00%
Tax loss on sale of METALfx	$ (1,259)	$ 0	$ (1,259)	$ 0
Tax loss on sale of METALfx	(5.40%)	0.00%	(0.80%)	0.00%
Valuation allowance	$ 1,245	$ 0	$ 1,245	$ 0
Valuation allowance	5.30%	0.00%	0.70%	0.00%
Settlement of equity awards	$ 0	$ 0	$ 612	$ (990)
Settlement of equity awards	0.00%	0.00%	0.40%	(1.00%)
Other	$ 525	$ 589	$ 2,046	$ (1,493)
Other	2.20%	1.90%	1.20%	(1.60%)
Income Tax Expense (Benefit)	$ (1,741)	$ 5,209	$ 28,276	$ 15,919
Effective Income Tax Rate	(7.50%)	16.90%	16.70%	16.50%
Income Tax Related
Effective Tax Rate Reconciliation [Line Items]
Regulatory liabilities	$ 23,079		$ 23,079		$ 27,997
Income Tax Related | IDAHO
Effective Tax Rate Reconciliation [Line Items]
Regulatory liabilities	$ 6,400		$ 6,400