Condensed Consolidated Statements of Income (unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Total revenues	$ 597,810	$ 575,389
Operating expenses:
Fuel, purchased power and cost of natural gas sold	248,779	247,639
Operations and maintenance	123,913	116,096
Depreciation, depletion and amortization	51,028	48,590
Taxes - property and production	13,519	13,300
Other operating expenses	440	1,490
Total operating expenses	437,679	427,115
Operating income	160,131	148,274
Interest charges -
Interest expense incurred net of amounts capitalized (including amortization of debt issuance costs, premiums and discounts)	(35,974)	(35,438)
Allowance for funds used during construction - borrowed	958	133
Interest income	299	310
Allowance for funds used during construction - equity	48	68
Other income (expense), net	(837)	(172)
Total other income (expense)	(35,506)	(35,099)
Income before income taxes	124,625	113,175
Income tax benefit (expense)	(17,263)	25,802
Income from continuing operations	107,362	138,977
Net (loss) from discontinued operations	0	(2,343)
Net income	107,362	136,634
Net income attributable to noncontrolling interest	(3,554)	(3,630)
Net income from continuing operations	103,808	135,347
Net income available for common stock	$ 103,808	$ 133,004
Earnings Per Share, Basic [Abstract]
Income from continuing operations, Basic (usd per share)	$ 1.73	$ 2.54
(Loss) from discontinued operations, Basic (usd per share)	0	(0.05)
Earnings (loss) per share, Basic (usd per share)	1.73	2.49
Earnings Per Share, Diluted [Abstract]
Income from continuing operations, Diluted (usd per share)	1.73	2.50
(Loss) from discontinued operations, Diluted (usd per share)	0	(0.04)
Earnings (loss) per share, Diluted (usd per share)	$ 1.73	$ 2.46
Weighted Average Number of Shares Outstanding, Basic and Diluted [Abstract]
Basic (in shares)	59,920	53,319
Diluted (in shares)	60,060	54,122