Condensed Consolidated Statements of Income (unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Revenues	$ 537,050	$ 597,810
Operating expenses:
Fuel, purchased power and cost of natural gas sold	187,879	249,742
Operations and maintenance	125,466	123,584
Depreciation, depletion and amortization	56,402	51,028
Taxes - property and production	14,118	13,325
Total operating expenses	383,865	437,679
Operating income	153,185	160,131
Other income (expense):
Interest expense incurred net of amounts capitalized (including amortization of debt issuance costs, premiums and discounts)	(35,781)	(35,016)
Interest income	328	299
Impairment of investment	(6,859)	0
Other income (expense), net	2,353	(789)
Total other income (expense)	(39,959)	(35,506)
Income before income taxes	113,226	124,625
Income tax (expense)	(16,002)	(17,263)
Net income	97,224	107,362
Net income attributable to noncontrolling interest	(4,050)	(3,554)
Net income available for common stock	$ 93,174	$ 103,808
Earnings per share of common stock:
Earnings (loss) per share, Basic (usd per share)	$ 1.51	$ 1.73
Earnings (loss) per share, Diluted (usd per share)	$ 1.51	$ 1.73
Weighted Average Number of Shares Outstanding, Basic and Diluted [Abstract]
Basic (in shares)	61,778	59,920
Diluted (in shares)	61,856	60,060