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Income Tax Expense - Continuing operations effective income tax rate (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Income Tax Disclosure [Abstract]        
Income Before Income Taxes - Continuing Operations $ 9,598 $ 7,418 $ 30,718 $ 18,061
Tax Computed at Company's Net Composite Federal and State Statutory Rate (39%) 3,743 2,893 11,980 7,044
Increases (Decreases) in Tax from:        
Federal Production Tax Credits (PTCs) (1,841) (1,831) (3,430) (3,818)
Reversal of Accrued Interest on Removal of Cost Capitalization Audit Issue       (676)
North Dakota Wind Tax Credit Amortization - Net of Federal Taxes (216) (149) (439) (371)
Corporate Owned Life Insurance (92) (13) (394) (385)
Medicare Part D Subsidy 4 (194)   (391)
Employee Stock Ownership Plan Dividend Deduction (188) (191) (378) (381)
Deferred Tax Asset Reduction - North Dakota due to Tax Rate Decrease 365   365  
Other Items - Net 319 2 276 (37)
Income Tax Expense - Continuing Operations $ 2,094 $ 517 $ 7,980 $ 985
Effective Income Tax Rate - Continuing Operations 21.80% 7.00% 26.00% 5.50%