Segment financial information - Schedule (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment financial information
Revenues	$ 799,180	$ 833,239	$ 795,417	$ 792,014	$ 853,424	$ 800,899	$ 798,800	$ 834,380	$ 3,219,850	$ 3,287,503	$ 3,420,917
Depreciation and amortization									293,533	282,306	270,405
Interest income									19,362	9,105	0
Interest expense									127,207	125,532	103,402
Income (loss) from continuing operations before income taxes									(1,791,595)	182,300	201,954
Income tax benefit									(470,962)	34,534	38,915
Income (loss) from continuing operations									(1,320,633)	147,766	163,039
Preferred stock dividends of subsidiaries									1,890	1,890	1,890
Income (loss) from continuing operations for common stock	29,166	(123,180)	(1,249,697)	21,188	45,558	29,753	34,406	36,159	(1,322,523)	145,876	161,149
Capital expenditures									344,251	442,727	339,333
Assets	8,931,416				17,243,821				8,931,416	17,243,821
Assets					7,677,372					7,677,372
Electric utility
Segment financial information
Revenues											3,408,583
Operating Segments
Segment financial information
Revenues											3,420,917
Operating Segments | Electric utility
Segment financial information
Revenues	796,174	829,617	792,331	788,578	849,982	794,987	794,191	830,361	3,206,700	3,269,521	3,408,587
Depreciation and amortization									282,970	270,195	260,744
Interest income									6,633	6,454
Interest expense									82,082	86,140	76,416
Income (loss) from continuing operations before income taxes									(1,663,914)	247,140	240,600
Income tax benefit									(439,547)	51,193	49,676
Income (loss) from continuing operations									(1,224,367)	195,947	190,924
Preferred stock dividends of subsidiaries									1,995	1,995	1,995
Income (loss) from continuing operations for common stock									(1,226,362)	193,952	188,929
Capital expenditures									329,479	438,775	329,457
Assets	7,613,604								7,613,604
Assets					7,283,554					7,283,554
All Other
Segment financial information
Revenues	3,006	$ 3,622	$ 3,086	$ 3,436	3,442	$ 5,912	$ 4,609	$ 4,019	13,150	17,982	12,330
Depreciation and amortization									10,563	12,111	9,661
Interest income									12,729	2,651
Interest expense									45,125	39,392	26,986
Income (loss) from continuing operations before income taxes									(127,681)	(64,840)	(38,646)
Income tax benefit									(31,415)	(16,659)	(10,761)
Income (loss) from continuing operations									(96,266)	(48,181)	(27,885)
Preferred stock dividends of subsidiaries									(105)	(105)	(105)
Income (loss) from continuing operations for common stock									(96,161)	(48,076)	(27,780)
Capital expenditures									14,772	3,952	9,876
Assets	$ 1,317,812								1,317,812
Assets					$ 393,818					$ 393,818
Asset impairment charges									$ 35,000
Intersegment eliminations
Segment financial information
Revenues											(4)
Intersegment eliminations | Electric utility
Segment financial information
Revenues											4
Corporate And Eliminations
Segment financial information
Revenues											$ 12,334