STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Regulated Operating Revenue	$ 273,287	$ 290,560	$ 801,455	$ 951,560
Cost of natural gas	62,510	82,588	288,649	447,664
Operating expenses
Operations and maintenance	103,517	101,482	208,356	209,757
Depreciation and amortization	47,387	44,943	94,900	88,789
General taxes	15,265	14,656	31,738	30,840
Total operating expenses	166,169	161,081	334,994	329,386
Operating income	44,608	46,891	177,812	174,510
Other expense, net	2,394	(865)	(3,394)	(436)
Interest expense, net	(15,843)	(15,399)	(31,536)	(31,185)
Income before income taxes	31,159	30,627	142,882	142,889
Income taxes	(5,834)	(6,157)	(25,880)	(24,759)
Net income	$ 25,325	$ 24,470	$ 117,002	$ 118,130
Earnings per share
Basic	$ 0.48	$ 0.46	$ 2.21	$ 2.23
Diluted	$ 0.48	$ 0.46	$ 2.20	$ 2.22
Weighted Average Number of Shares Outstanding, Basic [Abstract]
Basic	53,053	52,890	53,030	52,858
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Diluted	53,264	53,215	53,266	53,210
Dividends declared per share of stock	$ 0.54	$ 0.50	$ 1.08	$ 1.00
Retained Earnings [Member]
Operating expenses
Net income	$ 25,325	$ 24,470