Condensed Consolidated Statements of Income (unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Revenues	$ 346,590	$ 325,548	$ 1,210,554	$ 1,257,246
Operating expenses:
Fuel, purchased power and cost of natural gas sold	71,686	73,544	331,194	413,486
Operations and maintenance	122,759	116,583	365,533	365,116
Depreciation, depletion and amortization	56,348	51,884	169,413	154,507
Taxes - property and production	13,563	12,986	42,062	39,454
Total operating expenses	264,356	254,997	908,202	972,563
Operating income	82,234	70,551	302,352	284,683
Other income (expense):
Interest expense incurred net of amounts capitalized (including amortization of debt issuance costs, premiums and discounts)	(36,521)	(34,000)	(108,067)	(103,677)
Interest income	480	513	1,028	1,208
Impairment of investment	0	(19,741)	(6,859)	(19,741)
Other income (expense), net	(1,193)	580	(703)	55
Total other income (expense)	(37,234)	(52,648)	(114,601)	(122,155)
Income before income taxes	45,000	17,903	187,751	162,528
Income tax (expense)	(4,651)	(2,508)	(25,484)	(22,078)
Net income	40,349	15,395	162,267	140,450
Net income attributable to noncontrolling interest	(4,066)	(3,655)	(11,844)	(10,319)
Net income available for common stock	$ 36,283	$ 11,740	$ 150,423	$ 130,131
Earnings per share of common stock:
Earnings (loss) per share, Basic (usd per share)	$ 0.58	$ 0.19	$ 2.41	$ 2.15
Earnings (loss) per share, Diluted (usd per share)	$ 0.58	$ 0.19	$ 2.41	$ 2.15
Weighted Average Number of Shares Outstanding, Basic and Diluted [Abstract]
Basic (in shares)	62,575	60,976	62,310	60,458
Diluted (in shares)	62,630	61,104	62,362	60,578