STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Regulated Operating Revenue	$ 248,563	$ 238,280	$ 1,200,123	$ 1,169,265
Cost of natural gas	49,607	51,256	497,271	495,834
Operating expenses
Operations and maintenance	100,486	96,443	310,243	302,103
Depreciation and amortization	45,471	40,344	134,260	118,991
General taxes	14,222	13,996	45,062	44,763
Total operating expenses	160,179	150,783	489,565	465,857
Operating income	38,777	36,241	213,287	207,574
Other expense, net	(1,397)	(1,929)	(1,833)	(6,287)
Interest expense, net	(15,783)	(12,365)	(46,968)	(36,720)
Income before income taxes	21,597	21,947	164,486	164,567
Income taxes	(4,140)	(5,671)	(28,899)	(37,037)
Net income	$ 17,457	$ 16,276	$ 135,587	$ 127,530
Earnings per share
Basic	$ 0.33	$ 0.31	$ 2.56	$ 2.42
Diluted	$ 0.33	$ 0.31	$ 2.55	$ 2.41
Weighted Average Number of Shares Outstanding, Basic [Abstract]
Basic	52,933	52,736	52,883	52,678
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Diluted	53,267	53,112	53,229	52,969
Dividends declared per share of stock	$ 0.50	$ 0.46	$ 1.50	$ 1.38