Segment financial information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Segment financial information
Revenues from external customers	$ 831,229	$ 867,720	$ 2,412,023	$ 2,536,848
Total revenues	831,229	867,720	2,412,023	2,536,848
Income (loss) before income taxes	71,548	73,981	189,077	196,165
Income taxes (benefit)	22,841	25,804	65,157	69,926
Net income	48,707	48,177	123,920	126,239
Preferred stock dividends of subsidiaries	471	471	1,417	1,417
Net income for common stock	48,236	47,706	122,503	124,822
Assets	10,429,012		10,429,012		10,149,132
Electric utility
Segment financial information
Revenues from external customers	766,109	801,089	2,216,058	2,340,202
Intersegment revenues (eliminations)	6	6	18	55
Total revenues	766,115	801,095	2,216,076	2,340,257
Income (loss) before income taxes	57,373	61,268	144,212	154,976
Income taxes (benefit)	19,058	22,395	51,777	58,429
Net income	38,315	38,873	92,435	96,547
Preferred stock dividends of subsidiaries	498	498	1,496	1,496
Net income for common stock	37,817	38,375	90,939	95,051
Assets	5,269,758		5,269,758		5,108,793
Bank
Segment financial information
Revenues from external customers	65,058	66,596	195,841	196,569
Total revenues	65,058	66,596	195,841	196,569
Income (loss) before income taxes	22,808	21,627	69,265	66,964
Income taxes (benefit)	7,532	7,419	23,915	22,690
Net income	15,276	14,208	45,350	44,274
Net income for common stock	15,276	14,208	45,350	44,274
Assets	5,159,372		5,159,372		5,041,673
Other
Segment financial information
Revenues from external customers	62	35	124	77
Intersegment revenues (eliminations)	(6)	(6)	(18)	(55)
Total revenues	56	29	106	22
Income (loss) before income taxes	(8,633)	(8,914)	(24,400)	(25,775)
Income taxes (benefit)	(3,749)	(4,010)	(10,535)	(11,193)
Net income	(4,884)	(4,904)	(13,865)	(14,582)
Preferred stock dividends of subsidiaries	(27)	(27)	(79)	(79)
Net income for common stock	(4,857)	(4,877)	(13,786)	(14,503)
Assets	$ (118)		$ (118)		$ (1,334)