STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Regulated Operating Revenue	$ 528,168	$ 661,000
Cost of natural gas	226,139	365,076
Operating expenses
Operations and maintenance	104,839	108,275
Depreciation and amortization	47,513	43,846
General taxes	16,473	16,184
Total operating expenses	168,825	168,305
Operating income	133,204	127,619
Other expense, net	(5,788)	429
Interest expense, net	(15,693)	(15,786)
Income before income taxes	111,723	112,262
Income taxes	(20,046)	(18,602)
Net income	$ 91,677	$ 93,660
Earnings per share
Basic	$ 1.73	$ 1.77
Diluted	$ 1.72	$ 1.76
Weighted Average Number of Shares Outstanding, Basic [Abstract]
Basic	53,007	52,825
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Diluted	53,268	53,206
Dividends declared per share of stock	$ 0.54	$ 0.50
Retained Earnings [Member]
Operating expenses
Net income	$ 91,677	$ 93,660