Segment financial information (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2013 USD ($)	Jun. 30, 2013 USD ($)	Mar. 31, 2013 USD ($)	Dec. 31, 2014 USD ($) entity	Dec. 31, 2013 USD ($)		Dec. 31, 2012 USD ($)
Segment financial information
Revenues	$ 790,040	$ 867,096	$ 798,657	$ 783,749	$ 832,503	$ 829,168	$ 794,567	$ 782,232	$ 3,239,542	$ 3,238,470		$ 3,374,995
Depreciation and amortization									179,557	164,728		158,347
Interest expense									87,160	85,556		89,443
Income (loss) before income taxes									261,922	247,747		217,407
Income taxes (benefit)									91,712	84,341		76,859
Net income	33,630	48,286	41,894	46,400	39,486	48,707	41,061	34,152	170,210	163,406	[1]	140,548	[1]
Preferred stock dividends of subsidiaries									1,890	1,890		1,890
Net income for common stock	33,157	$ 47,815	$ 41,421	$ 45,927	39,013	$ 48,236	$ 40,588	$ 33,679	168,320	161,516		138,658
Capital expenditures									364,826	389,438	[1]	370,715	[1]
Assets	11,184,161				10,340,044				$ 11,184,161	10,340,044		10,149,132
Electric utility
Segment financial information
Number of entities involved | entity									3
Revenues									$ 2,987,323	2,980,172		3,109,439
Depreciation and amortization									172,797	159,102		151,496
Interest expense									64,757	59,279		62,055
Income (loss) before income taxes									220,361	194,041		162,319
Income taxes (benefit)									80,725	69,117		61,048
Net income									139,636	124,924		101,271
Preferred stock dividends of subsidiaries									1,995	1,995		1,995
Net income for common stock									137,641	122,929		99,276
Capital expenditures									336,679	378,044		355,326
Assets	5,590,457				5,087,129				5,590,457	5,087,129		5,108,793
Bank
Segment financial information
Revenues									252,497	258,147		265,539
Depreciation and amortization									5,399	4,230		5,334
Interest expense									10,808	10,077		11,292
Income (loss) before income taxes									75,619	87,059		89,021
Income taxes (benefit)									24,127	29,525		30,384
Net income									51,492	57,534		58,637
Preferred stock dividends of subsidiaries									0	0		0
Net income for common stock									51,492	57,534		58,637
Capital expenditures									28,073	11,193		14,979
Assets	5,565,241				5,243,824				5,565,241	5,243,824		5,041,673
Other
Segment financial information
Revenues									(278)	151		17
Depreciation and amortization									1,361	1,396		1,517
Interest expense									11,595	16,200		16,096
Income (loss) before income taxes									(34,058)	(33,353)		(33,933)
Income taxes (benefit)									(13,140)	(14,301)		(14,573)
Net income									(20,918)	(19,052)		(19,360)
Preferred stock dividends of subsidiaries									(105)	(105)		(105)
Net income for common stock									(20,813)	(18,947)		(19,255)
Capital expenditures									74	201		410
Assets	$ 28,463				$ 9,091				28,463	9,091		(1,334)
Operating Segments
Segment financial information
Revenues									3,239,542	3,238,470		3,374,995
Operating Segments | Electric utility
Segment financial information
Revenues									2,987,299	2,980,139		3,109,353
Operating Segments | Bank
Segment financial information
Revenues									252,497	258,147		265,539
Operating Segments | Other
Segment financial information
Revenues									(254)	184		103
Intersegment eliminations
Segment financial information
Revenues									0	0		0
Intersegment eliminations | Electric utility
Segment financial information
Revenues									24	33		86
Intersegment eliminations | Bank
Segment financial information
Revenues									0	0		0
Intersegment eliminations | Other
Segment financial information
Revenues									$ (24)	$ (33)		$ (86)

[1] As restated - See Note 1, “Summary of significant accounting policies - Revision and restatements of previously issued financial statements.”