Segment financial information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Segment financial information
Revenues from external customers	$ 784,064	$ 814,860
Total revenues	784,064	814,860
Income (loss) before income taxes	52,814	60,087
Income taxes (benefit)	18,662	21,298
Net income	34,152	38,789
Preferred stock dividends of subsidiaries	473	473
Net income for common stock	33,679	38,316
Assets	10,298,569	10,149,132
Electric utility
Segment financial information
Revenues from external customers	719,267	749,574
Intersegment revenues (eliminations)	6	36
Total revenues	719,273	749,610
Income (loss) before income taxes	39,322	45,207
Income taxes (benefit)	14,394	17,408
Net income	24,928	27,799
Preferred stock dividends of subsidiaries	499	499
Net income for common stock	24,429	27,300
Assets	5,174,235	5,108,793
Bank
Segment financial information
Revenues from external customers	64,756	65,252
Total revenues	64,756	65,252
Income (loss) before income taxes	21,752	23,464
Income taxes (benefit)	7,597	7,587
Net income	14,155	15,877
Net income for common stock	14,155	15,877
Assets	5,116,385	5,041,673
Other
Segment financial information
Revenues from external customers	41	34
Intersegment revenues (eliminations)	(6)	(36)
Total revenues	35	(2)
Income (loss) before income taxes	(8,260)	(8,584)
Income taxes (benefit)	(3,329)	(3,697)
Net income	(4,931)	(4,887)
Preferred stock dividends of subsidiaries	(26)	(26)
Net income for common stock	(4,905)	(4,861)
Assets	$ 7,949	$ (1,334)