Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017		Jun. 30, 2018	Jun. 30, 2017
Operating revenues:
Operating revenues:	$ 1,064,597	$ 1,067,639		$ 2,040,890	$ 2,005,565
Operating expenses:
Operation and maintenance:	822,084	821,948		1,422,940	1,380,491
Purchased natural gas sold	56,228	57,668		238,196	250,617
Depreciation, depletion and amortization	53,553	51,658		106,282	102,983
Taxes, other than income	40,757	40,953		89,610	88,391
Electric fuel and purchased power	17,983	16,752		40,494	38,638
Total operating expenses	990,605	988,979		1,897,522	1,861,120
Operating income	73,992	78,660		143,368	144,445
Other income	1,599	1,801		2,181	4,144
Interest expense	20,800	20,766		41,246	41,068
Income before income taxes	54,791	59,695		104,303	107,521
Income taxes	10,716	15,290	[1]	18,267	27,478	[1]
Income from continuing operations	44,075	44,405		86,036	80,043
Income (loss) from discontinued operations, net of tax (Note 10)	(273)	(3,190)	[1]	203	(1,504)	[1]
Net income	43,802	41,215		86,239	78,539
Loss on redemption of preferred stocks	0	600		0	600
Dividends declared on preferred stocks	0	0		0	171
Earnings on common stock	$ 43,802	$ 40,615		$ 86,239	$ 77,768
Earnings per common share - basic:
Earnings before discontinued operations	$ 0.22	$ 0.22		$ 0.44	$ 0.41
Discontinued operations, net of tax	0	(0.01)		0	(0.01)
Earnings per common share - basic	0.22	0.21		0.44	0.40
Earnings per common share - diluted:
Earnings before discontinued operations	0.22	0.22		0.44	0.40
Discontinued operations, net of tax	0	(0.01)		0	0
Earnings per common share - diluted	0.22	0.21		0.44	0.40
Dividends declared per common share	$ 0.1975	$ 0.1925		$ 0.3950	$ 0.3850
Weighted average common shares outstanding - basic	195,524	195,304		195,415	195,304
Weighted average common shares outstanding - diluted	196,169	195,973		196,077	195,993
Regulated operations [Member]
Operating revenues:
Operating revenues:	$ 226,684	$ 225,485		$ 651,143	$ 659,100
Operating expenses:
Operation and maintenance:	83,928	78,125		170,042	157,677
Nonregulated operations [Member]
Operating revenues:
Operating revenues:	837,913	842,154		1,389,747	1,346,465
Operating expenses:
Operation and maintenance:	$ 738,156	$ 743,823		$ 1,252,898	$ 1,222,814

[1] Includes the eliminations for the presentation of income tax adjustments between continuing and discontinued operations.