STATEMENTS OF INCOME (UNAUDITED) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Income Statement [Abstract]        
Operating revenues $ 554,926 $ 530,752 $ 1,386,210 $ 1,377,566
Operating expenses        
Electric fuel and purchased power 297,587 287,476 757,537 776,301
Operating and maintenance expenses 71,699 72,036 202,410 219,760
Demand side management program expenses 5,663 3,726 12,279 10,155
Depreciation and amortization 42,026 35,422 123,250 107,911
Taxes (other than income taxes) 15,589 15,016 46,417 43,472
Total operating expenses 432,564 413,676 1,141,893 1,157,599
Operating income 122,362 117,076 244,317 219,967
Other income, net 137 103 563 203
Allowance for funds used during construction — equity 2,632 2,085 7,348 5,578
Interest charges and financing costs        
Interest charges — includes other financing costs of $828, $789, $2,461 and $2,334, respectively 23,343 21,779 67,350 63,737
Allowance for funds used during construction — debt (1,422) (1,204) (4,146) (3,402)
Total interest charges and financing costs 21,921 20,575 63,204 60,335
Income before income taxes 103,210 98,689 189,024 165,413
Income taxes 34,864 36,874 65,944 60,775
Net income $ 68,346 $ 61,815 $ 123,080 $ 104,638