Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues
Revenues	$ 2,874,601	$ 2,860,849	$ 2,555,625
Expenses
Total expenses	2,525,927	2,527,493	2,209,392
Operating income (loss)
Operating income	348,674	333,356	346,233
Retirement defined benefits expense—other than service costs	(2,806)	(5,962)	(7,942)
Interest expense, net – other than on deposit liabilities and other bank borrowings	(90,899)	(88,677)	(78,972)
Allowance for borrowed funds used during construction	4,453	4,867	4,778
Allowance for equity funds used during construction	11,987	10,877	12,483
Income before income taxes	271,409	254,461	276,580
Income taxes	51,637	50,797	109,393
Net income (loss)	219,772	203,664	167,187
Preferred stock dividends of subsidiaries	1,890	1,890	1,890
Net income for common stock	$ 217,882	$ 201,774	$ 165,297
Basic earnings per common share (in dollars per share)	$ 2.00	$ 1.85	$ 1.52
Diluted earnings per common share (in dollars per share)	$ 1.99	$ 1.85	$ 1.52
Weighted-average number of common shares outstanding (in shares)	108,949	108,855	108,749
Net effect of potentially dilutive shares (in shares)	458	291	184
Weighted-average shares assuming dilution (in shares)	109,407	109,146	108,933
Electric utility
Revenues
Revenues	$ 2,545,942	$ 2,546,525	$ 2,257,566
Expenses
Total expenses	2,291,564	2,304,864	1,994,042
Operating income (loss)
Operating income	254,378	241,661	263,524
Income before income taxes	197,140	180,426	205,145
Income taxes	38,305	34,778	83,199
Net income (loss)	158,835	145,648	121,946
Preferred stock dividends of subsidiaries	1,995	1,995	1,995
Net income for common stock	156,840	143,653	119,951
Bank
Revenues
Revenues	328,570	314,275	297,640
Expenses
Total expenses	217,008	206,040	198,104
Operating income (loss)
Operating income	111,562	108,235	99,536
Income before income taxes	112,034	106,578	98,716
Income taxes	23,061	24,069	31,719
Net income (loss)	88,973	82,509	66,997
Preferred stock dividends of subsidiaries	0	0	0
Net income for common stock	88,973	82,509	66,997
Other
Revenues
Revenues	89	49	419
Expenses
Total expenses	17,355	16,589	17,246
Operating income (loss)
Operating income	(17,266)	(16,540)	(16,827)
Income before income taxes	(37,765)	(32,543)	(27,281)
Income taxes	(9,729)	(8,050)	(5,525)
Net income (loss)	(28,036)	(24,493)	(21,756)
Preferred stock dividends of subsidiaries	(105)	(105)	(105)
Net income for common stock	$ (27,931)	$ (24,388)	$ (21,651)