STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Regulated Operating Revenue	$ 661,000	$ 638,464
Cost of natural gas	365,076	350,419
Operating expenses
Operations and maintenance	108,275	102,665
Depreciation and amortization	43,846	38,890
General taxes	16,184	16,200
Total operating expenses	168,305	157,755
Operating income	127,619	130,290
Other expense, net	429	(2,164)
Interest expense, net	(15,786)	(12,352)
Income before income taxes	112,262	115,774
Income taxes	(18,602)	(24,939)
Net income	$ 93,660	$ 90,835
Earnings per share
Basic	$ 1.77	$ 1.73
Diluted	$ 1.76	$ 1.72
Weighted Average Number of Shares Outstanding, Basic [Abstract]
Basic	52,825	52,604
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Diluted	53,206	52,897
Dividends declared per share of stock	$ 0.50	$ 0.46