STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gross Margin
Revenues	$ 1,539,633	$ 1,427,232	$ 1,547,692
Cost of natural gas	614,501	541,797	705,959
Net margin	925,132	885,435	841,733
Operating expenses
Operations and maintenance	416,542	417,142	414,476
Depreciation and amortization	151,889	143,829	133,023
General taxes	57,225	55,344	55,105
Total operating expenses	625,656	616,315	602,604
Operating income	299,476	269,120	239,129
Other income	4,217	1,447	263
Other expense	(1,490)	(1,490)	(2,813)
Interest expense, net	(46,065)	(43,739)	(44,570)
Income before income taxes	256,138	225,338	192,009
Income taxes	(93,143)	(85,243)	(72,979)
Net income	$ 162,995	$ 140,095	$ 119,030
Earnings per share
Basic	$ 3.10	$ 2.67	$ 2.26
Diluted	$ 3.08	$ 2.65	$ 2.24
Average shares (thousands)
Basic	52,527	52,453	52,578
Diluted	52,979	52,963	53,254
Dividends declared per share of stock	$ 1.68	$ 1.40	$ 1.20