Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating revenues:
Operating revenues	$ 4,531,552	$ 4,443,351	$ 4,128,828
Operating expenses:
Operation and maintenance:	3,256,121	3,135,466	2,893,510
Purchased natural gas sold	404,153	430,954	382,753
Depreciation, depletion and amortization	220,205	207,486	216,318
Taxes, other than income	168,638	166,673	151,826
Electric fuel and purchased power	80,712	78,724	75,512
Total operating expenses	4,129,829	4,019,303	3,719,919
Operating income	401,723	424,048	408,909
Other income (expense)	(238)	8,767	5,167
Interest expense	84,614	82,788	87,848
Income before income taxes	316,871	350,027	326,228
Income taxes	47,485	65,041	93,132
Income from continuing operations	269,386	284,986	233,096
Income (loss) from discontinued operations, net of tax (Note 4)	2,932	(3,783)	(300,354)
Net income (loss)	272,318	281,203	(67,258)
Loss from discontinued operations attributable to noncontrolling interest (Note 4)	0	0	(131,691)
Loss on redemption of preferred stocks (Note 10)	0	600	0
Dividends declared on preferred stocks	0	171	685
Earnings on common stock	$ 272,318	$ 280,432	$ 63,748
Earnings per common share - basic:
Earnings before discontinued operations	$ 1.38	$ 1.46	$ 1.19
Discontinued operations attributable to the Company, net of tax	0.01	(0.02)	(0.86)
Earnings per common share - basic	1.39	1.44	0.33
Earnings per common share - diluted:
Earnings before discontinued operations	1.38	1.45	1.19
Discontinued operations attributable to the Company, net of tax	0.01	(0.02)	(0.86)
Earnings per common share - diluted	$ 1.39	$ 1.43	$ 0.33
Weighted average common shares outstanding - basic	195,720	195,304	195,299
Weighted average common shares outstanding - diluted	196,150	195,687	195,618
Regulated:
Operating revenues:
Operating revenues	$ 1,213,227	$ 1,244,759	$ 1,141,454
Operating expenses:
Operation and maintenance:	340,331	326,687	312,211
Nonregulated:
Operating revenues:
Operating revenues	3,318,325	3,198,592	2,987,374
Operating expenses:
Operation and maintenance:	$ 2,915,790	$ 2,808,779	$ 2,581,299