Segment financial information (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) entity	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Segment financial information
Revenues	$ 624,032	$ 717,176	$ 623,912	$ 637,862	$ 790,040	$ 867,096	$ 798,657	$ 783,749	$ 2,602,982	$ 3,239,542	$ 3,238,470
Depreciation and amortization									195,585	183,044	167,385
Interest expense, net									88,476	87,160	85,556
Income (loss) before income taxes									254,788	265,598	249,836
Income taxes (benefit)									93,021	95,579	86,237
Net income (loss)	42,793	51,144	35,491	32,339	33,726	48,279	41,754	46,260	161,767	170,019	163,599
Preferred stock dividends of subsidiaries									1,890	1,890	1,890
Net income for common stock	42,320	$ 50,673	$ 35,018	$ 31,866	33,253	$ 47,808	$ 41,281	$ 45,787	159,877	168,129	161,709
Capital expenditures									363,804	364,826	389,438
Assets	11,790,196				11,185,142				$ 11,790,196	11,185,142	10,340,906
Electric utility
Segment financial information
Number of entities involved | entity									3
Revenues									$ 2,335,166	2,987,323	2,980,172
Depreciation and amortization									186,319	176,284	161,759
Interest expense, net									66,370	64,757	59,279
Income (loss) before income taxes									217,131	220,361	194,041
Income taxes (benefit)									79,422	80,725	69,117
Net income (loss)									137,709	139,636	124,924
Preferred stock dividends of subsidiaries									1,995	1,995	1,995
Net income for common stock									135,714	137,641	122,929
Capital expenditures									350,161	336,679	378,044
Assets	5,680,054				5,557,542				5,680,054	5,557,542	5,066,427
Bank
Segment financial information
Revenues									267,733	252,497	258,147
Depreciation and amortization									7,928	5,399	4,230
Interest expense, net									11,326	10,808	10,077
Income (loss) before income taxes									83,812	79,295	89,148
Income taxes (benefit)									29,082	27,994	31,421
Net income (loss)									54,730	51,301	57,727
Preferred stock dividends of subsidiaries									0	0	0
Net income for common stock									54,730	51,301	57,727
Capital expenditures									13,470	28,073	11,193
Assets	6,014,755				5,566,222				6,014,755	5,566,222	5,244,686
Other
Segment financial information
Revenues									83	(278)	151
Depreciation and amortization									1,338	1,361	1,396
Interest expense, net									10,780	11,595	16,200
Income (loss) before income taxes									(46,155)	(34,058)	(33,353)
Income taxes (benefit)									(15,483)	(13,140)	(14,301)
Net income (loss)									(30,672)	(20,918)	(19,052)
Preferred stock dividends of subsidiaries									(105)	(105)	(105)
Net income for common stock									(30,567)	(20,813)	(18,947)
Capital expenditures									173	74	201
Assets	$ 95,387				$ 61,378				95,387	61,378	29,793
Operating Segments
Segment financial information
Revenues									2,602,982	3,239,542	3,238,470
Operating Segments | Electric utility
Segment financial information
Revenues									2,335,135	2,987,299	2,980,139
Operating Segments | Bank
Segment financial information
Revenues									267,733	252,497	258,147
Operating Segments | Other
Segment financial information
Revenues									114	(254)	184
Intersegment eliminations
Segment financial information
Revenues									0	0	0
Intersegment eliminations | Electric utility
Segment financial information
Revenues									31	24	33
Intersegment eliminations | Bank
Segment financial information
Revenues									0	0	0
Intersegment eliminations | Other
Segment financial information
Revenues									$ (31)	$ (24)	$ (33)