Segment financial information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Segment financial information
Revenues from external customers	$ 867,720	$ 886,355	$ 2,536,848	$ 2,391,307
Total revenues	867,720	886,355	2,536,848	2,391,307
Income (loss) before income taxes	73,981	76,769	196,165	163,122
Income taxes (benefit)	25,804	27,894	69,926	57,700
Net income	48,177	48,875	126,239	105,422
Preferred stock dividends of subsidiaries	471	471	1,417	1,417
Net income for common stock	47,706	48,404	124,822	104,005
Assets	9,904,827		9,904,827		9,594,477
Electric utility
Segment financial information
Revenues from external customers	801,089	820,218	2,340,202	2,194,219
Intersegment revenues (eliminations)	6	36	55	108
Total revenues	801,095	820,254	2,340,257	2,194,327
Income (loss) before income taxes	61,268	62,244	154,976	122,114
Income taxes (benefit)	22,395	23,787	58,429	46,446
Net income	38,873	38,457	96,547	75,668
Preferred stock dividends of subsidiaries	498	498	1,496	1,496
Net income for common stock	38,375	37,959	95,051	74,172
Assets	4,961,715		4,961,715		4,674,007
Bank
Segment financial information
Revenues from external customers	66,596	66,100	196,569	197,731
Total revenues	66,596	66,100	196,569	197,731
Income (loss) before income taxes	21,627	23,166	66,964	68,699
Income taxes (benefit)	7,419	7,709	22,690	24,196
Net income	14,208	15,457	44,274	44,503
Net income for common stock	14,208	15,457	44,274	44,503
Assets	4,870,660		4,870,660		4,909,974
Other
Segment financial information
Revenues from external customers	35	37	77	(643)
Intersegment revenues (eliminations)	(6)	(36)	(55)	(108)
Total revenues	29	1	22	(751)
Income (loss) before income taxes	(8,914)	(8,641)	(25,775)	(27,691)
Income taxes (benefit)	(4,010)	(3,602)	(11,193)	(12,942)
Net income	(4,904)	(5,039)	(14,582)	(14,749)
Preferred stock dividends of subsidiaries	(27)	(27)	(79)	(79)
Net income for common stock	(4,877)	(5,012)	(14,503)	(14,670)
Assets	$ (9,738)		$ (9,738)		$ 10,496