Segment financial information (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Segment financial information
Revenues from external customers	$ 886,355	$ 694,541	$ 2,391,307	$ 1,969,245
Total revenues	886,355	694,541	2,391,307	1,969,245
Income (loss) before income taxes	76,769	53,305	163,122	141,931
Income taxes (benefit)	27,894	20,385	57,700	51,677
Net income (loss)	48,875	32,920	105,422	90,254
Preferred stock dividends of subsidiaries	471	471	1,417	1,417
Net income (loss) for common stock	48,404	32,449	104,005	88,837
Tangible assets at the end of the period	9,175,121	8,996,455	9,175,121	8,996,455	8,996,455
Electric utility
Segment financial information
Revenues from external customers	820,218	623,090	2,194,219	1,755,213
Intersegment revenues (eliminations)	36	36	108	119
Total revenues	820,254	623,126	2,194,327	1,755,332
Income (loss) before income taxes	62,244	37,197	122,114	95,063
Income taxes (benefit)	23,787	14,719	46,446	35,893
Net income (loss)	38,457	22,478	75,668	59,170
Preferred stock dividends of subsidiaries	498	498	1,496	1,496
Net income (loss) for common stock	37,959	21,980	74,172	57,674
Tangible assets at the end of the period	4,350,759	4,285,680	4,350,759	4,285,680	4,285,680
Bank
Segment financial information
Revenues from external customers	66,100	71,429	197,731	213,975
Total revenues	66,100	71,429	197,731	213,975
Income (loss) before income taxes	23,166	24,359	68,699	71,842
Income taxes (benefit)	7,709	9,066	24,196	26,682
Net income (loss)	15,457	15,293	44,503	45,160
Net income (loss) for common stock	15,457	15,293	44,503	45,160
Tangible assets at the end of the period	4,811,421	4,707,870	4,811,421	4,707,870	4,707,870
Other
Segment financial information
Revenues from external customers	37	22	(643)	57
Intersegment revenues (eliminations)	(36)	(36)	(108)	(119)
Total revenues	1	(14)	(751)	(62)
Income (loss) before income taxes	(8,641)	(8,251)	(27,691)	(24,974)
Income taxes (benefit)	(3,602)	(3,400)	(12,942)	(10,898)
Net income (loss)	(5,039)	(4,851)	(14,749)	(14,076)
Preferred stock dividends of subsidiaries	(27)	(27)	(79)	(79)
Net income (loss) for common stock	(5,012)	(4,824)	(14,670)	(13,997)
Tangible assets at the end of the period	$ 12,941	$ 2,905	$ 12,941	$ 2,905	$ 2,905