Segment financial information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Segment financial information
Total revenues	$ 798,657	$ 794,567	$ 1,582,406	$ 1,576,799
Income (loss) before income taxes	64,163	63,723	135,236	115,762
Income taxes (benefit)	22,269	22,662	46,942	40,549
Net income	41,894	41,061	88,294	75,213
Preferred stock dividends of subsidiaries	473	473	946	946
Net income for common stock	41,421	40,588	87,348	74,267
Assets	10,617,143		10,617,143		10,340,044
Electric utility
Segment financial information
Total revenues	738,429	728,525	1,458,491	1,445,966
Income (loss) before income taxes	55,126	46,525	112,292	85,072
Income taxes (benefit)	20,397	17,333	41,644	30,952
Net income	34,729	29,192	70,648	54,120
Preferred stock dividends of subsidiaries	499	499	998	998
Net income for common stock	34,230	28,693	69,650	53,122
Assets	5,199,841		5,199,841		5,087,129
Bank
Segment financial information
Total revenues	60,616	66,027	124,235	130,783
Income (loss) before income taxes	17,048	24,705	38,672	46,457
Income taxes (benefit)	5,372	8,786	12,457	16,383
Net income	11,676	15,919	26,215	30,074
Net income for common stock	11,676	15,919	26,215	30,074
Assets	5,418,127		5,418,127		5,243,824
Other
Segment financial information
Total revenues	(388)	15	(320)	50
Income (loss) before income taxes	(8,011)	(7,507)	(15,728)	(15,767)
Income taxes (benefit)	(3,500)	(3,457)	(7,159)	(6,786)
Net income	(4,511)	(4,050)	(8,569)	(8,981)
Preferred stock dividends of subsidiaries	(26)	(26)	(52)	(52)
Net income for common stock	(4,485)	(4,024)	(8,517)	(8,929)
Assets	(825)		(825)		9,091
Revenues from external customers
Segment financial information
Total revenues			1,582,406	1,576,799
Revenues from external customers | Electric utility
Segment financial information
Total revenues	738,423	728,519	1,458,479	1,445,954
Revenues from external customers | Bank
Segment financial information
Total revenues			124,235	130,783
Revenues from external customers | Other
Segment financial information
Total revenues	(382)	21	(308)	62
Intersegment revenues (eliminations) | Electric utility
Segment financial information
Total revenues	6	6	12	12
Intersegment revenues (eliminations) | Other
Segment financial information
Total revenues	$ (6)	$ (6)	$ (12)	$ (12)