Segment financial information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment financial information
Revenues from external customers									$ 3,242,335	$ 2,664,982	$ 2,309,590
Total revenues	851,028	886,355	794,319	710,633	695,737	694,541	655,664	619,040	3,242,335	2,664,982	2,309,590
Depreciation and amortization									167,470	159,128	156,671
Interest expenses									96,575	101,887	119,873
Income (loss) before income taxes									216,052	183,247	128,824
Income taxes (benefit)									75,932	67,822	43,923
Net income									140,120	115,425	84,901
Preferred stock dividends of subsidiaries									1,890	1,890	1,890
Net income for common stock	34,225	48,404	27,139	28,462	24,698	32,449	29,262	27,126	138,230	113,535	83,011
Capital expenditures									235,116	182,125	288,879
Tangible assets at the end of the period	9,502,314				8,996,455				9,502,314	8,996,455	8,838,612
Electric utility
Segment financial information
Revenues from external customers									2,978,547	2,382,211	2,034,834
Intersegment revenues (eliminations)									143	155	175
Total revenues									2,978,690	2,382,366	2,035,009
Depreciation and amortization									160,353	157,432	154,578
Interest expenses									60,031	61,510	57,944
Income (loss) before income taxes									163,565	125,452	129,217
Income taxes (benefit)									61,584	46,868	47,776
Net income									101,981	78,584	81,441
Preferred stock dividends of subsidiaries									1,995	1,995	1,995
Net income for common stock									99,986	76,589	79,446
Capital expenditures									226,022	174,344	286,445
Tangible assets at the end of the period	4,671,942				4,285,680				4,671,942	4,285,680	3,978,392
Bank
Segment financial information
Revenues from external customers									264,407	282,693	274,719
Total revenues									264,407	282,693	274,719
Depreciation and amortization									5,909	749	1,309
Interest expenses									14,469	20,349	43,543
Income (loss) before income taxes									91,536	92,512	31,705
Income taxes (benefit)									31,693	34,056	9,938
Net income									59,843	58,456	21,767
Net income for common stock									59,843	58,456	21,767
Capital expenditures									8,984	7,709	2,188
Tangible assets at the end of the period	4,819,557				4,707,870				4,819,557	4,707,870	4,854,595
Other
Segment financial information
Revenues from external customers									(619)	78	37
Intersegment revenues (eliminations)									(143)	(155)	(175)
Total revenues									(762)	(77)	(138)
Depreciation and amortization									1,208	947	784
Interest expenses									22,075	20,028	18,386
Income (loss) before income taxes									(39,049)	(34,717)	(32,098)
Income taxes (benefit)									(17,345)	(13,102)	(13,791)
Net income									(21,704)	(21,615)	(18,307)
Preferred stock dividends of subsidiaries									(105)	(105)	(105)
Net income for common stock									(21,599)	(21,510)	(18,202)
Capital expenditures									110	72	246
Tangible assets at the end of the period	$ 10,815				$ 2,905				$ 10,815	$ 2,905	$ 5,625