Segment financial information (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Segment financial information
Total revenues		$ 637,862	$ 783,749
Income (loss) before income taxes		52,318	71,981
Income taxes (benefit)		19,979	25,721
Net income	[1]	32,339	46,260
Preferred stock dividends of subsidiaries		473	473
Net income for common stock		31,866	45,787
Assets		11,273,447		$ 11,185,142
Electric utility
Segment financial information
Total revenues		573,442	720,062
Income (loss) before income taxes		43,223	57,166
Income taxes (benefit)		15,850	21,247
Net income		27,373	35,919
Preferred stock dividends of subsidiaries		499	499
Net income for common stock		26,874	35,420
Assets		5,535,500		5,590,457
Bank
Segment financial information
Total revenues		64,348	63,619
Income (loss) before income taxes		20,631	22,532
Income taxes (benefit)		7,156	8,133
Net income		13,475	14,399
Net income for common stock		13,475	14,399
Assets		5,724,877		5,566,222
Other
Segment financial information
Total revenues		72	68
Income (loss) before income taxes		(11,536)	(7,717)
Income taxes (benefit)		(3,027)	(3,659)
Net income		(8,509)	(4,058)
Preferred stock dividends of subsidiaries		(26)	(26)
Net income for common stock		(8,483)	(4,032)
Assets		13,070		$ 28,463
Revenues from external customers
Segment financial information
Total revenues		637,862	783,749
Revenues from external customers | Electric utility
Segment financial information
Total revenues		573,431	720,056
Revenues from external customers | Bank
Segment financial information
Total revenues		64,348	63,619
Revenues from external customers | Other
Segment financial information
Total revenues		83	74
Intersegment revenues (eliminations) | Electric utility
Segment financial information
Total revenues		11	6
Intersegment revenues (eliminations) | Other
Segment financial information
Total revenues		$ (11)	$ (6)

[1] As restated - See Note 1, “Basis of presentation - Restatement of previously issued financial statements.”