Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues
Revenues	$ 2,602,982	$ 3,239,542	$ 3,238,470
Expenses
Total expenses	2,280,429	2,906,942	2,920,962
Operating income (loss)
Operating income	322,553	332,600	317,508
Interest expense, net – other than on deposit liabilities and other bank borrowings	(77,150)	(76,352)	(75,479)
Allowance for borrowed funds used during construction	2,457	2,579	2,246
Allowance for equity funds used during construction	6,928	6,771	5,561
Income before income taxes	254,788	265,598	249,836
Income taxes	93,021	95,579	86,237
Net income (loss)	161,767	170,019	163,599
Preferred stock dividends of subsidiaries	1,890	1,890	1,890
Net income for common stock	$ 159,877	$ 168,129	$ 161,709
Basic earnings per common share (in dollars per share)	$ 1.50	$ 1.65	$ 1.63
Diluted earnings per common share (in dollars per share)	1.50	1.63	1.62
Dividends per common share (in dollars per share)	$ 1.24	$ 1.24	$ 1.24
Weighted-average number of common shares outstanding (in shares)	106,418	101,968	98,968
Net effect of potentially dilutive shares (in shares)	303	969	655
Adjusted weighted-average shares (in shares)	106,721	102,937	99,623
Electric utility
Revenues
Revenues	$ 2,335,166	$ 2,987,323	$ 2,980,172
Expenses
Total expenses	2,061,050	2,711,555	2,734,659
Operating income (loss)
Operating income	274,116	275,768	245,513
Income before income taxes	217,131	220,361	194,041
Income taxes	79,422	80,725	69,117
Net income (loss)	137,709	139,636	124,924
Preferred stock dividends of subsidiaries	1,995	1,995	1,995
Net income for common stock	135,714	137,641	122,929
Bank
Revenues
Revenues	267,733	252,497	258,147
Expenses
Total expenses	183,921	173,202	169,001
Operating income (loss)
Operating income	83,812	79,295	89,146
Income before income taxes	83,812	79,295	89,148
Income taxes	29,082	27,994	31,421
Net income (loss)	54,730	51,301	57,727
Preferred stock dividends of subsidiaries	0	0	0
Net income for common stock	54,730	51,301	57,727
Other
Revenues
Revenues	83	(278)	151
Expenses
Total expenses	35,458	22,185	17,302
Operating income (loss)
Operating income	(35,375)	(22,463)	(17,151)
Income before income taxes	(46,155)	(34,058)	(33,353)
Income taxes	(15,483)	(13,140)	(14,301)
Net income (loss)	(30,672)	(20,918)	(19,052)
Preferred stock dividends of subsidiaries	(105)	(105)	(105)
Net income for common stock	$ (30,567)	$ (20,813)	$ (18,947)