Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
Revenues	$ 2,555,625	$ 2,380,654	$ 2,602,982
Expenses
Total expenses	2,217,334	2,032,479	2,280,429
Operating income (loss)
Operating income	338,291	348,175	322,553
Merger termination fee	0	90,000	0
Interest expense, net – other than on deposit liabilities and other bank borrowings	(78,972)	(75,803)	(77,150)
Allowance for borrowed funds used during construction	4,778	3,144	2,457
Allowance for equity funds used during construction	12,483	8,325	6,928
Income before income taxes	276,580	373,841	254,788
Income taxes	109,393	123,695	93,021
Net income (loss)	167,187	250,146	161,767
Preferred stock dividends of subsidiaries	1,890	1,890	1,890
Net income for common stock	$ 165,297	$ 248,256	$ 159,877
Basic earnings per common share (in dollars per share)	$ 1.52	$ 2.30	$ 1.50
Diluted earnings per common share (in dollars per share)	$ 1.52	$ 2.29	$ 1.50
Weighted-average number of common shares outstanding (in shares)	108,749	108,102	106,418
Net effect of potentially dilutive shares (in shares)	184	207	303
Weighted-average shares assuming dilution (in shares)	108,933	108,309	106,721
Electric utility
Revenues
Revenues	$ 2,257,566	$ 2,094,368	$ 2,335,166
Expenses
Total expenses	2,000,045	1,809,900	2,061,050
Operating income (loss)
Operating income	257,521	284,468	274,116
Income before income taxes	205,145	229,113	217,131
Income taxes	83,199	84,801	79,422
Net income (loss)	121,946	144,312	137,709
Preferred stock dividends of subsidiaries	1,995	1,995	1,995
Net income for common stock	119,951	142,317	135,714
Bank
Revenues
Revenues	297,640	285,924	267,733
Expenses
Total expenses	198,924	198,572	183,921
Operating income (loss)
Operating income	98,716	87,352	83,812
Income before income taxes	98,716	87,352	83,812
Income taxes	31,719	30,073	29,082
Net income (loss)	66,997	57,279	54,730
Preferred stock dividends of subsidiaries	0	0	0
Net income for common stock	66,997	57,279	54,730
Other
Revenues
Revenues	419	362	83
Expenses
Total expenses	18,365	24,007	35,458
Operating income (loss)
Operating income	(17,946)	(23,645)	(35,375)
Income before income taxes	(27,281)	57,376	(46,155)
Income taxes	(5,525)	8,821	(15,483)
Net income (loss)	(21,756)	48,555	(30,672)
Preferred stock dividends of subsidiaries	(105)	(105)	(105)
Net income for common stock	$ (21,651)	$ 48,660	$ (30,567)