STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Regulated Operating Revenue	$ 290,560	$ 292,521	$ 951,560	$ 930,985
Cost of natural gas	82,588	94,159	447,664	444,578
Operating expenses
Operations and maintenance	101,482	102,995	209,757	205,660
Depreciation and amortization	44,943	39,757	88,789	78,647
General taxes	14,656	14,567	30,840	30,767
Total operating expenses	161,081	157,319	329,386	315,074
Operating income	46,891	41,043	174,510	171,333
Other expense, net	(865)	(2,194)	(436)	(4,358)
Interest expense, net	(15,399)	(12,003)	(31,185)	(24,355)
Income before income taxes	30,627	26,846	142,889	142,620
Income taxes	(6,157)	(6,427)	(24,759)	(31,366)
Net income	$ 24,470	$ 20,419	$ 118,130	$ 111,254
Earnings per share
Basic	$ 0.46	$ 0.39	$ 2.23	$ 2.11
Diluted	$ 0.46	$ 0.39	$ 2.22	$ 2.10
Weighted Average Number of Shares Outstanding, Basic [Abstract]
Basic	52,890	52,692	52,858	52,648
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Diluted	53,215	52,899	53,210	52,898
Dividends declared per share of stock	$ 0.50	$ 0.46	$ 1.00	$ 0.92