STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Gross Margin
Revenues	$ 1,427,232	$ 1,547,692	$ 1,818,906
Cost of natural gas	541,797	705,959	991,949
Net margin	885,435	841,733	826,957
Operating expenses
Operations and maintenance	417,142	414,476	420,686
Depreciation and amortization	143,829	133,023	125,722
General taxes	55,344	55,105	55,255
Total operating expenses	616,315	602,604	601,663
Operating income	269,120	239,129	225,294
Other income	1,447	263	1,625
Other expense	(1,490)	(2,813)	(2,949)
Interest expense, net	(43,739)	(44,570)	(45,842)
Income before income taxes	225,338	192,009	178,128
Income taxes	(85,243)	(72,979)	(68,338)
Net income	140,095	119,030	109,790
Net Income (Loss) Available to Common Stockholders, Diluted	$ 140,095	$ 119,030	$ 109,790
Earnings per share
Basic	$ 2.67	$ 2.26	$ 2.10
Diluted	$ 2.65	$ 2.24	$ 2.07
Average shares (thousands)
Basic	52,453	52,578	52,364
Diluted	52,963	53,254	52,946
Dividends declared per share of stock	$ 1.40	$ 1.20	$ 0.84