Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Operating revenues:
Operating revenues:		$ 1,272,548	$ 1,208,567	$ 3,278,112	$ 3,112,730
Operating expenses:
Operation and maintenance:		972,909	920,540	2,351,053	2,237,486
Electric fuel and purchased power		18,906	16,800	57,544	54,725
Purchased natural gas sold		33,319	34,321	283,936	242,795
Depreciation, depletion and amortization		52,155	54,094	155,138	163,226
Taxes, other than income		38,882	36,128	127,273	116,864
Total operating expenses		1,116,171	1,061,883	2,974,944	2,815,096
Operating income		156,377	146,684	303,168	297,634
Other income		1,011	1,741	2,809	3,662
Interest expense		20,909	22,278	61,978	67,365
Income before income taxes		136,479	126,147	243,999	233,931
Income taxes		46,930	37,761	74,406	67,381
Income from continuing operations		89,549	88,386	169,593	166,550
Loss from discontinued operations, net of tax (Note 8)	[1]	(2,198)	(5,400)	(3,702)	(299,538)
Net income (loss)		87,351	82,986	165,891	(132,988)
Loss from discontinued operations attributable to noncontrolling interest (Note 8)		0	0	0	(131,691)
Loss on redemption of preferred stocks		0	0	600	0
Dividends declared on preferred stocks		0	171	171	514
Earnings (loss) on common stock		$ 87,351	$ 82,815	$ 165,120	$ (1,811)
Earnings (loss) per common share - basic:
Earnings before discontinued operations		$ 0.46	$ 0.45	$ 0.86	$ 0.85
Discontinued operations attributable to the Company, net of tax		(0.01)	(0.03)	(0.01)	(0.86)
Earnings (loss) per common share - basic		0.45	0.42	0.85	(0.01)
Earnings (loss) per common share - diluted:
Earnings before discontinued operations		0.46	0.45	0.86	0.85
Discontinued operations attributable to the Company, net of tax		(0.01)	(0.03)	(0.02)	(0.86)
Earnings (loss) per common share - diluted		0.45	0.42	0.84	(0.01)
Dividends declared per common share		$ 0.1925	$ 0.1875	$ 0.5775	$ 0.5625
Weighted average common shares outstanding - basic		195,304	195,304	195,304	195,298
Weighted average common shares outstanding - diluted		195,783	195,811	195,922	195,794
Regulated operations [Member]
Operating revenues:
Operating revenues:		$ 206,936	$ 192,079	$ 866,035	$ 783,997
Operating expenses:
Operation and maintenance:		79,293	77,662	235,306	229,364
Nonregulated operations [Member]
Operating revenues:
Operating revenues:		1,065,612	1,016,488	2,412,077	2,328,733
Operating expenses:
Operation and maintenance:		$ 893,616	$ 842,878	$ 2,115,747	$ 2,008,122

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