Statements Of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Operating revenues	$ 297,470	$ 323,225	$ 720,648	$ 698,899	$ 908,685	$ 875,801
Energy expenses:
Fuel	57,889	54,355	143,668	131,817	185,589	171,877
Purchased and interchanged power	17,430	24,459	47,824	47,715	59,836	58,823
Cost of services, energy services	75,319	78,814	191,492	179,532	245,425	230,700
Operating revenues net of energy expenses	222,151	244,411	529,156	519,367	663,260	645,101
Other operating expenses:
Other operations	60,692	64,373	176,650	179,577	239,087	243,912
Maintenance	11,944	14,064	53,349	52,005	68,090	67,456
Depreciation and amortization	22,565	15,952	66,994	63,097	88,214	85,841
Taxes other than income taxes	21,213	20,165	54,208	50,297	69,444	65,189
Utilities operating expense	116,414	114,554	351,201	344,976	464,835	462,398
Operating income	105,737	129,857	177,955	174,391	198,425	182,703
Other income (deductions):
Allowance for equity funds used during construction	668	1,398	2,209	5,867	3,365	8,089
Investment and interest income, net	3,715	3,773	14,487	10,293	18,277	15,237
Miscellaneous non-operating income	242	272	366	1,073	585	1,598
Miscellaneous non-operating deductions	(1,067)	(1,312)	(2,337)	(2,668)	(3,368)	(4,219)
Nonoperating income	3,558	4,131	14,725	14,565	18,859	20,705
Interest charges (credits):
Interest on long-term debt and revolving credit facility	18,215	18,324	54,989	53,221	73,312	69,629
Other interest	728	268	1,910	1,102	2,111	1,474
Capitalized interest	(1,193)	(1,243)	(3,831)	(3,738)	(5,083)	(4,948)
Public Utilities, Allowance for Funds Used During Construction, Additions	671	1,131	2,173	4,164	2,992	5,736
Interest expense	17,079	16,218	50,895	46,421	67,348	60,419
Income before income taxes	92,216	117,770	141,785	142,535	149,936	142,989
Income tax expense	32,532	43,134	50,024	51,423	52,519	51,229
Net income	$ 59,684	$ 74,636	$ 91,761	$ 91,112	$ 97,417	$ 91,760
Basic earnings per share	$ 1.470	$ 1.840	$ 2.260	$ 2.250	$ 2.400	$ 2.270
Diluted earnings per share	1.470	1.840	2.260	2.250	2.400	2.270
Dividends declared per share of common stock	$ 0.335	$ 0.310	$ 0.980	$ 0.915	$ 1.290	$ 1.210
Weighted average number of shares outstanding	40,427,589	40,363,819	40,408,100	40,344,834	40,398,022	40,332,835
Weighted average number of shares and dilutive potential shares outstanding	40,551,315	40,425,942	40,516,777	40,395,811	40,498,642	40,380,443