Segment financial information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Segment financial information
Revenues	$ 783,749	$ 782,232
Income (loss) before income taxes	71,073	52,039
Income taxes (benefit)	24,673	17,887
Net income	46,400	34,152
Preferred stock dividends of subsidiaries	473	473
Net income for common stock	45,927	33,679
Assets	10,458,070		10,340,044
Assets			10,340,044
Electric utility
Segment financial information
Revenues	720,062	717,441
Income (loss) before income taxes	57,166	38,547
Income taxes (benefit)	21,247	13,619
Net income	35,919	24,928
Preferred stock dividends of subsidiaries	499	499
Net income for common stock	35,420	24,429
Assets	5,082,589
Assets			5,087,129
Bank
Segment financial information
Revenues	63,619	64,756
Income (loss) before income taxes	21,624	21,752
Income taxes (benefit)	7,085	7,597
Net income	14,539	14,155
Net income for common stock	14,539	14,155
Assets	5,371,483
Assets			5,243,824
Other
Segment financial information
Revenues	68	35
Income (loss) before income taxes	(7,717)	(8,260)
Income taxes (benefit)	(3,659)	(3,329)
Net income	(4,058)	(4,931)
Preferred stock dividends of subsidiaries	(26)	(26)
Net income for common stock	(4,032)	(4,905)
Assets	3,998
Assets			9,091
Revenues from external customers
Segment financial information
Revenues	783,749	782,232
Revenues from external customers | Electric utility
Segment financial information
Revenues	720,056	717,435
Revenues from external customers | Bank
Segment financial information
Revenues	63,619	64,756
Revenues from external customers | Other
Segment financial information
Revenues	74	41
Intersegment revenues (eliminations) | Electric utility
Segment financial information
Revenues	6	6
Intersegment revenues (eliminations) | Other
Segment financial information
Revenues	$ (6)	$ (6)