Segment financial information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Segment financial information
Revenues from external customers	$ 814,860	$ 710,633
Total revenues	814,860	710,633
Income (loss) before income taxes	60,087	44,999
Income taxes (benefit)	21,298	16,064
Net income	38,789	28,935
Preferred stock dividends of subsidiaries	473	473
Net income for common stock	38,316	28,462
Tangible assets at the end of the period	9,550,762	9,510,541
Electric utility
Segment financial information
Revenues from external customers	749,574	645,299
Intersegment revenues (eliminations)	36	36
Total revenues	749,610	645,335
Income (loss) before income taxes	45,207	31,267
Income taxes (benefit)	17,408	11,579
Net income	27,799	19,688
Preferred stock dividends of subsidiaries	499	499
Net income for common stock	27,300	19,189
Tangible assets at the end of the period	4,656,064	4,671,942
Bank
Segment financial information
Revenues from external customers	65,252	65,313
Total revenues	65,252	65,313
Income (loss) before income taxes	23,464	21,727
Income taxes (benefit)	7,587	7,876
Net income	15,877	13,851
Net income for common stock	15,877	13,851
Tangible assets at the end of the period	4,880,927	4,827,784
Other
Segment financial information
Revenues from external customers	34	21
Intersegment revenues (eliminations)	(36)	(36)
Total revenues	(2)	(15)
Income (loss) before income taxes	(8,584)	(7,995)
Income taxes (benefit)	(3,697)	(3,391)
Net income	(4,887)	(4,604)
Preferred stock dividends of subsidiaries	(26)	(26)
Net income for common stock	(4,861)	(4,578)
Tangible assets at the end of the period	$ 13,771	$ 10,815