Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues
Revenues	$ 2,579,775	$ 2,873,948	$ 2,860,849
Expenses
Total expenses	2,268,282	2,525,927	2,527,493
Operating income (loss)
Operating income	311,493	348,021	333,356
Retirement defined benefits expense—other than service costs	(3,210)	(2,806)	(5,962)
Interest expense, net – other than on deposit liabilities and other bank borrowings	(88,694)	(90,899)	(88,677)
Allowance for borrowed funds used during construction	2,992	4,453	4,867
Allowance for equity funds used during construction	8,768	11,987	10,877
Gain on sale of investment securities, net	9,275	653	0
Income before income taxes	240,624	271,409	254,461
Income taxes	40,910	51,637	50,797
Net income (loss)	199,714	219,772	203,664
Preferred stock dividends of subsidiaries	1,890	1,890	1,890
Net income for common stock	$ 197,824	$ 217,882	$ 201,774
Basic earnings per common share (in dollars per share)	$ 1.81	$ 2.00	$ 1.85
Diluted earnings per common share (in dollars per share)	$ 1.81	$ 1.99	$ 1.85
Weighted-average number of common shares outstanding (in shares)	109,140	108,949	108,855
Net effect of potentially dilutive shares (in shares)	216	458	291
Weighted-average shares assuming dilution (in shares)	109,356	109,407	109,146
Electric utility
Revenues
Revenues	$ 2,265,320	$ 2,545,942	$ 2,546,525
Expenses
Total expenses	1,996,770	2,291,564	2,304,864
Operating income (loss)
Operating income	268,550	254,378	241,661
Income before income taxes	211,753	197,140	180,426
Income taxes	40,418	38,305	34,778
Net income (loss)	171,335	158,835	145,648
Preferred stock dividends of subsidiaries	1,995	1,995	1,995
Net income for common stock	169,340	156,840	143,653
Bank
Revenues
Revenues	313,511	327,917	314,275
Expenses
Total expenses	251,702	217,008	206,040
Operating income (loss)
Operating income	61,809	110,909	108,235
Income before income taxes	69,271	112,034	106,578
Income taxes	11,688	23,061	24,069
Net income (loss)	57,583	88,973	82,509
Preferred stock dividends of subsidiaries	0	0	0
Net income for common stock	57,583	88,973	82,509
Other
Revenues
Revenues	944	89	49
Expenses
Total expenses	19,810	17,355	16,589
Operating income (loss)
Operating income	(18,866)	(17,266)	(16,540)
Income before income taxes	(40,400)	(37,765)	(32,543)
Income taxes	(11,196)	(9,729)	(8,050)
Net income (loss)	(29,204)	(28,036)	(24,493)
Preferred stock dividends of subsidiaries	(105)	(105)	(105)
Net income for common stock	$ (29,099)	$ (27,931)	$ (24,388)