STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues	$ 1,633,731	$ 1,539,633	$ 1,427,232
Cost of natural gas	714,636	614,501	541,797
Operating expenses
Operations and maintenance	411,702	399,290	397,315
Depreciation and amortization	160,086	151,889	143,829
General taxes	58,878	57,225	55,344
Total operating expenses	630,666	608,404	596,488
Operating income	288,429	316,728	288,947
Other expense, net	(11,359)	(14,525)	(19,870)
Interest expense, net	(51,305)	(46,065)	(43,739)
Income before income taxes	225,765	256,138	225,338
Income taxes	(53,531)	(93,143)	(85,243)
Net income	$ 172,234	$ 162,995	$ 140,095
Earnings per share
Basic	$ 3.27	$ 3.10	$ 2.67
Diluted	$ 3.25	$ 3.08	$ 2.65
Average shares (thousands)
Basic	52,693	52,527	52,453
Diluted	53,029	52,979	52,963
Dividends declared per share of stock	$ 1.84	$ 1.68	$ 1.40