Segment financial information (Details) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Segment financial information
Revenues from external customers	$ 794,319	$ 655,664	$ 1,504,952	$ 1,274,704
Total revenues	794,319	655,664	1,504,952	1,274,704
Income (loss) before income taxes	41,354	45,748	86,353	88,626
Income taxes (benefit)	13,742	16,013	29,806	31,292
Net income (loss)	27,612	29,735	56,547	57,334
Preferred stock dividends of subsidiaries	473	473	946	946
Net income (loss) for common stock	27,139	29,262	55,601	56,388
Tangible assets at the end of the period	9,152,027		9,152,027		8,996,455
Electric utility
Segment financial information
Revenues from external customers	728,702	584,048	1,374,001	1,132,123
Intersegment revenues (eliminations)	36	47	72	83
Total revenues	728,738	584,095	1,374,073	1,132,206
Income (loss) before income taxes	28,603	28,354	59,870	57,866
Income taxes (benefit)	11,080	10,213	22,659	21,174
Net income (loss)	17,523	18,141	37,211	36,692
Preferred stock dividends of subsidiaries	499	499	998	998
Net income (loss) for common stock	17,024	17,642	36,213	35,694
Tangible assets at the end of the period	4,279,122		4,279,122		4,285,680
Bank
Segment financial information
Revenues from external customers	66,318	71,632	131,631	142,546
Total revenues	66,318	71,632	131,631	142,546
Income (loss) before income taxes	23,806	25,747	45,533	47,483
Income taxes (benefit)	8,611	9,616	16,487	17,616
Net income (loss)	15,195	16,131	29,046	29,867
Net income (loss) for common stock	15,195	16,131	29,046	29,867
Tangible assets at the end of the period	4,801,483		4,801,483		4,707,870
Other
Segment financial information
Revenues from external customers	(701)	(16)	(680)	35
Intersegment revenues (eliminations)	(36)	(47)	(72)	(83)
Total revenues	(737)	(63)	(752)	(48)
Income (loss) before income taxes	(11,055)	(8,353)	(19,050)	(16,723)
Income taxes (benefit)	(5,949)	(3,816)	(9,340)	(7,498)
Net income (loss)	(5,106)	(4,537)	(9,710)	(9,225)
Preferred stock dividends of subsidiaries	(26)	(26)	(52)	(52)
Net income (loss) for common stock	(5,080)	(4,511)	(9,658)	(9,173)
Tangible assets at the end of the period	$ 71,422		$ 71,422		$ 2,905