Segment financial information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Segment financial information
Revenues from external customers	$ 854,268	$ 794,319	$ 1,669,128	$ 1,504,952
Total revenues	854,268	794,319	1,669,128	1,504,952
Income (loss) before income taxes	62,097	41,354	122,184	86,353
Income taxes (benefit)	22,824	13,742	44,122	29,806
Net income	39,273	27,612	78,062	56,547
Preferred stock dividends of subsidiaries	473	473	946	946
Net income for common stock	38,800	27,139	77,116	55,601
Tangible assets at the end of the period	9,739,767		9,739,767		9,512,287
Electric utility
Segment financial information
Revenues from external customers	789,539	728,702	1,539,113	1,374,001
Intersegment revenues (eliminations)	13	36	49	72
Total revenues	789,552	728,738	1,539,162	1,374,073
Income (loss) before income taxes	48,501	28,603	93,708	59,870
Income taxes (benefit)	18,626	11,080	36,034	22,659
Net income	29,875	17,523	57,674	37,211
Preferred stock dividends of subsidiaries	499	499	998	998
Net income for common stock	29,376	17,024	56,676	36,213
Tangible assets at the end of the period	4,857,550		4,857,550		4,674,007
Bank
Segment financial information
Revenues from external customers	64,721	66,318	129,973	131,631
Total revenues	64,721	66,318	129,973	131,631
Income (loss) before income taxes	21,873	23,806	45,337	45,533
Income taxes (benefit)	7,684	8,611	15,271	16,487
Net income	14,189	15,195	30,066	29,046
Net income for common stock	14,189	15,195	30,066	29,046
Tangible assets at the end of the period	4,882,005		4,882,005		4,827,784
Other
Segment financial information
Revenues from external customers	8	(701)	42	(680)
Intersegment revenues (eliminations)	(13)	(36)	(49)	(72)
Total revenues	(5)	(737)	(7)	(752)
Income (loss) before income taxes	(8,277)	(11,055)	(16,861)	(19,050)
Income taxes (benefit)	(3,486)	(5,949)	(7,183)	(9,340)
Net income	(4,791)	(5,106)	(9,678)	(9,710)
Preferred stock dividends of subsidiaries	(26)	(26)	(52)	(52)
Net income for common stock	(4,765)	(5,080)	(9,626)	(9,658)
Tangible assets at the end of the period	$ 212		$ 212		$ 10,496