Segment financial information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment financial information
Revenues	$ 617,395	$ 646,055	$ 566,244	$ 550,960	$ 624,032	$ 717,176	$ 623,912	$ 637,862	$ 2,380,654	$ 2,602,982	$ 3,239,542
Depreciation and amortization									204,746	195,585	183,044
Interest expense, net									88,558	88,476	87,160
Income before income taxes									373,841	254,788	265,598
Income taxes									123,695	93,021	95,579
Net income	45,107	127,613	44,601	32,825	42,793	51,144	35,491	32,339	250,146	161,767	170,019
Preferred stock dividends of subsidiaries									1,890	1,890	1,890
Net income for common stock	44,634	$ 127,142	$ 44,128	$ 32,352	42,320	$ 50,673	$ 35,018	$ 31,866	248,256	159,877	168,129
Capital expenditures									330,043	363,804	364,826
Assets	12,425,506				11,782,018				12,425,506	11,782,018	11,177,143
Electric utility
Segment financial information
Revenues									2,094,368	2,335,166	2,987,323
Depreciation and amortization									193,996	186,319	176,284
Interest expense, net									66,824	66,370	64,757
Income before income taxes									229,113	217,131	220,361
Income taxes									84,801	79,422	80,725
Net income									144,312	137,709	139,636
Preferred stock dividends of subsidiaries									1,995	1,995	1,995
Net income for common stock									142,317	135,714	137,641
Capital expenditures									320,437	350,161	336,679
Assets	5,975,428				5,672,210				5,975,428	5,672,210	5,550,021
Bank
Segment financial information
Revenues									285,924	267,733	252,497
Depreciation and amortization									9,813	7,928	5,399
Interest expense, net									12,755	11,326	10,808
Income before income taxes									87,352	83,812	79,295
Income taxes									30,073	29,082	27,994
Net income									57,279	54,730	51,301
Preferred stock dividends of subsidiaries									0	0	0
Net income for common stock									57,279	54,730	51,301
Capital expenditures									9,394	13,470	28,073
Assets	6,421,357				6,014,755				6,421,357	6,014,755	5,566,222
Other
Segment financial information
Revenues									362	83	(278)
Depreciation and amortization									937	1,338	1,361
Interest expense, net									8,979	10,780	11,595
Income before income taxes									57,376	(46,155)	(34,058)
Income taxes									8,821	(15,483)	(13,140)
Net income									48,555	(30,672)	(20,918)
Preferred stock dividends of subsidiaries									(105)	(105)	(105)
Net income for common stock									48,660	(30,567)	(20,813)
Capital expenditures									212	173	74
Assets	$ 28,721				$ 95,053				28,721	95,053	60,900
Operating Segments
Segment financial information
Revenues									2,380,654	2,602,982	3,239,542
Operating Segments | Electric utility
Segment financial information
Revenues									2,094,224	2,335,135	2,987,299
Operating Segments | Bank
Segment financial information
Revenues									285,924	267,733	252,497
Operating Segments | Other
Segment financial information
Revenues									506	114	(254)
Intersegment eliminations
Segment financial information
Revenues									0	0	0
Intersegment eliminations | Electric utility
Segment financial information
Revenues									144	31	24
Intersegment eliminations | Bank
Segment financial information
Revenues									0	0	0
Intersegment eliminations | Other
Segment financial information
Revenues									$ (144)	$ (31)	$ (24)