Segment financial information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Segment financial information
Revenues from external customers	$ 796,730	$ 854,268	$ 1,580,794	$ 1,669,128
Total revenues	796,730	854,268	1,580,794	1,669,128
Income (loss) before income taxes	64,715	62,097	117,529	122,184
Income taxes (benefit)	23,654	22,824	42,316	44,122
Net income	41,061	39,273	75,213	78,062
Preferred stock dividends of subsidiaries	473	473	946	946
Net income for common stock	40,588	38,800	74,267	77,116
Assets	10,238,145		10,238,145		10,149,132
Electric utility
Segment financial information
Revenues from external customers	730,682	789,539	1,449,949	1,539,113
Intersegment revenues (eliminations)	6	13	12	49
Total revenues	730,688	789,552	1,449,961	1,539,162
Income (loss) before income taxes	47,517	48,501	86,839	93,708
Income taxes (benefit)	18,325	18,626	32,719	36,034
Net income	29,192	29,875	54,120	57,674
Preferred stock dividends of subsidiaries	499	499	998	998
Net income for common stock	28,693	29,376	53,122	56,676
Assets	5,161,819		5,161,819		5,108,793
Bank
Segment financial information
Revenues from external customers	66,027	64,721	130,783	129,973
Total revenues	66,027	64,721	130,783	129,973
Income (loss) before income taxes	24,705	21,873	46,457	45,337
Income taxes (benefit)	8,786	7,684	16,383	15,271
Net income	15,919	14,189	30,074	30,066
Net income for common stock	15,919	14,189	30,074	30,066
Assets	5,068,771		5,068,771		5,041,673
Other
Segment financial information
Revenues from external customers	21	8	62	42
Intersegment revenues (eliminations)	(6)	(13)	(12)	(49)
Total revenues	15	(5)	50	(7)
Income (loss) before income taxes	(7,507)	(8,277)	(15,767)	(16,861)
Income taxes (benefit)	(3,457)	(3,486)	(6,786)	(7,183)
Net income	(4,050)	(4,791)	(8,981)	(9,678)
Preferred stock dividends of subsidiaries	(26)	(26)	(52)	(52)
Net income for common stock	(4,024)	(4,765)	(8,929)	(9,626)
Assets	$ 7,555		$ 7,555		$ (1,334)