Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating revenues:
Operating revenues		$ 4,443,351	$ 4,128,828	$ 4,014,052
Operation and maintenance:
Operation and maintenance:		3,130,802	2,893,299	2,805,223
Purchased natural gas sold		430,954	382,753	450,114
Depreciation, depletion and amortization		207,486	216,318	211,747
Taxes, other than income		166,673	151,826	140,955
Electric fuel and purchased power		78,724	75,512	86,238
Total operating expenses		4,014,639	3,719,708	3,694,277
Operating income		428,712	409,120	319,775
Other income		4,103	4,956	18,457
Interest expense		82,788	87,848	91,179
Income before income taxes		350,027	326,228	247,053
Income taxes		65,041	93,132	70,664
Income from continuing operations		284,986	233,096	176,389
Loss from discontinued operations, net of tax (Note 2)	[1]	(3,783)	(300,354)	(834,080)
Net income (loss)		281,203	(67,258)	(657,691)
Loss from discontinued operations attributable to noncontrolling interest (Note 2)		0	(131,691)	(35,256)
Loss on redemption of preferred stocks (Note 8)		600	0	0
Dividends declared on preferred stocks		171	685	685
Earnings (loss) on common stock		$ 280,432	$ 63,748	$ (623,120)
Earnings (loss) per common share - basic:
Earnings before discontinued operations		$ 1.46	$ 1.19	$ 0.90
Discontinued operations attributable to the Company, net of tax		(0.02)	(0.86)	(4.10)
Earnings (loss) per common share - basic		1.44	0.33	(3.20)
Earnings (loss) per common share - diluted:
Earnings before discontinued operations		1.45	1.19	0.90
Discontinued operations attributable to the Company, net of tax		(0.02)	(0.86)	(4.10)
Earnings (loss) per common share - diluted		$ 1.43	$ 0.33	$ (3.20)
Weighted average common shares outstanding - basic		195,304	195,299	194,928
Weighted average common shares outstanding - diluted		195,687	195,618	194,986
Regulated operations [Member]
Operating revenues:
Operating revenues		$ 1,244,759	$ 1,141,454	$ 1,149,038
Operation and maintenance:
Operation and maintenance:		323,120	312,404	278,171
Nonregulated operations [Member]
Operating revenues:
Operating revenues		3,198,592	2,987,374	2,865,014
Operation and maintenance:
Operation and maintenance:		$ 2,807,682	$ 2,580,895	$ 2,527,052

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