Condensed Consolidated Statements of Income (unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Revenues	$ 326,914	$ 333,888	$ 863,964	$ 931,698
Operating expenses:
Fuel, purchased power and cost of natural gas sold	71,629	90,200	259,508	339,942
Operations and maintenance	117,308	124,950	242,774	248,534
Depreciation, depletion and amortization	56,663	51,595	113,065	102,623
Taxes - property and production	14,381	13,142	28,499	26,467
Total operating expenses	259,981	279,887	643,846	717,566
Operating income	66,933	54,001	220,118	214,132
Other income (expense):
Interest expense incurred net of amounts capitalized (including amortization of debt issuance costs, premiums and discounts)	(35,765)	(34,660)	(71,546)	(69,676)
Interest income	220	396	548	695
Impairment of investment	0	0	(6,859)	0
Other income (expense), net	(1,863)	263	490	(526)
Total other income (expense)	(37,408)	(34,001)	(77,367)	(69,507)
Income before income taxes	29,525	20,000	142,751	144,625
Income tax (expense)	(4,831)	(2,307)	(20,833)	(19,570)
Net income	24,694	17,693	121,918	125,055
Net income attributable to noncontrolling interest	(3,728)	(3,110)	(7,778)	(6,664)
Net income available for common stock	$ 20,966	$ 14,583	$ 114,140	$ 118,391
Earnings per share of common stock:
Earnings (loss) per share, Basic (usd per share)	$ 0.34	$ 0.24	$ 1.84	$ 1.97
Earnings (loss) per share, Diluted (usd per share)	$ 0.33	$ 0.24	$ 1.83	$ 1.96
Weighted Average Number of Shares Outstanding, Basic and Diluted [Abstract]
Basic (in shares)	62,573	60,467	62,175	60,195
Diluted (in shares)	62,617	60,606	62,230	60,333