Income Tax Expense - Continuing operations effective income tax rate (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Tax Disclosure [Abstract]
Income Before Income Taxes - Continuing Operations	$ 19,757	$ 22,230	$ 60,378	$ 57,749
Tax Computed at Company's Net Composite Federal and State Statutory Rate (39%)	7,705	8,670	23,547	22,522
Increases (Decreases) in Tax from:
Federal Production Tax Credits	(1,423)	(1,437)	(4,994)	(5,147)
R&D Tax Credits	(223)	2	(445)	(7)
North Dakota Wind Tax Credit Amortization - Net of Federal Taxes	(212)	(212)	(637)	(637)
Employee Stock Ownership Plan Dividend Deduction	(157)	(171)	(472)	(514)
Corporate Owned Life Insurance	(92)	185	(664)	(39)
Investment Tax Credits	(87)	(143)	(262)	(428)
Adjustment for Uncertain Tax Positions	(57)	281	(31)	367
AFUDC Equity	(51)	(144)	(238)	(369)
Section 199 Domestic Production Activities Deduction	(9)	(362)	(207)	(1,087)
Other Items - Net	(231)	(148)	141	(59)
Income Tax Expense - Continuing Operations	$ 5,163	$ 6,521	$ 15,738	$ 14,602
Effective Income Tax Rate - Continuing Operations	26.10%	29.30%	26.10%	25.30%