STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Regulated Operating Revenue	$ 244,640	$ 248,563	$ 1,046,095	$ 1,200,123
Cost of natural gas	40,485	49,607	329,134	497,271
Operating expenses
Operations and maintenance	100,285	100,486	308,641	310,243
Depreciation and amortization	47,998	45,471	142,898	134,260
General taxes	15,193	14,222	46,931	45,062
Total operating expenses	163,476	160,179	498,470	489,565
Operating income	40,679	38,777	218,491	213,287
Other expense, net	198	(1,397)	(3,196)	(1,833)
Interest expense, net	(15,542)	(15,783)	(47,078)	(46,968)
Income before income taxes	25,335	21,597	168,217	164,486
Income taxes	(4,256)	(4,140)	(30,136)	(28,899)
Net income	$ 21,079	$ 17,457	$ 138,081	$ 135,587
Earnings per share
Basic	$ 0.40	$ 0.33	$ 2.60	$ 2.56
Diluted	$ 0.39	$ 0.33	$ 2.59	$ 2.55
Weighted Average Number of Shares Outstanding, Basic [Abstract]
Basic	53,190	52,933	53,084	52,883
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Diluted	53,408	53,267	53,313	53,229
Dividends declared per share of stock	$ 0.54	$ 0.50	$ 1.62	$ 1.50
Retained Earnings [Member]
Operating expenses
Net income	$ 21,079	$ 17,457