Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenues
Total revenues	$ 2,850,379	$ 2,579,775	$ 2,873,948
Expenses
Total expenses	2,464,313	2,268,282	2,525,927
Operating income (loss)
Operating income	386,066	311,493	348,021
Retirement defined benefits credit (expense)—other than service costs	5,848	(3,210)	(2,806)
Interest expense, net – other than on deposit liabilities and other bank borrowings	(94,363)	(88,694)	(90,899)
Allowance for borrowed funds used during construction	3,250	2,992	4,453
Allowance for equity funds used during construction	9,534	8,768	11,987
Gain on sale of investment securities, net	528	9,275	653
Income before income taxes	310,863	240,624	271,409
Income taxes	62,807	40,910	51,637
Net income (loss)	248,056	199,714	219,772
Preferred stock dividends of subsidiaries	1,890	1,890	1,890
Net income for common stock	$ 246,166	$ 197,824	$ 217,882
Basic earnings per common share (in dollars per share)	$ 2.25	$ 1.81	$ 2.00
Diluted earnings per common share (in dollars per share)	$ 2.25	$ 1.81	$ 1.99
Weighted-average number of common shares outstanding (in shares)	109,282	109,140	108,949
Net effect of potentially dilutive shares (in shares)	298	216	458
Weighted-average shares assuming dilution (in shares)	109,580	109,356	109,407
Electric utility
Revenues
Total revenues	$ 2,539,636	$ 2,265,320	$ 2,545,942
Expenses
Total expenses	2,260,078	1,996,770	2,291,564
Operating income (loss)
Operating income	279,558	268,550	254,378
Income before income taxes	223,785	211,753	197,140
Income taxes	44,148	40,418	38,305
Net income (loss)	179,637	171,335	158,835
Preferred stock dividends of subsidiaries	1,995	1,995	1,995
Net income for common stock	177,642	169,340	156,840
Bank
Revenues
Total revenues	306,398	313,511	327,917
Expenses
Total expenses	178,195	251,702	217,008
Operating income (loss)
Operating income	128,203	61,809	110,909
Income before income taxes	130,559	69,271	112,034
Income taxes	29,325	11,688	23,061
Net income (loss)	101,234	57,583	88,973
Preferred stock dividends of subsidiaries	0	0	0
Net income for common stock	101,234	57,583	88,973
Other
Revenues
Total revenues	4,345	944	89
Expenses
Total expenses	26,040	19,810	17,355
Operating income (loss)
Operating income	(21,695)	(18,866)	(17,266)
Income before income taxes	(43,481)	(40,400)	(37,765)
Income taxes	(10,666)	(11,196)	(9,729)
Net income (loss)	(32,815)	(29,204)	(28,036)
Preferred stock dividends of subsidiaries	(105)	(105)	(105)
Net income for common stock	$ (32,710)	$ (29,099)	$ (27,931)