Segment financial information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment financial information
Total revenues	$ 632,281	$ 566,244	$ 1,223,843	$ 1,117,204
Income (loss) before income taxes	59,626	70,911	111,208	122,037
Income taxes (benefit)	20,492	26,310	37,408	44,611
Net income	39,134	44,601	73,800	77,426
Preferred stock dividends of subsidiaries	473	473	946	946
Net income for common stock	38,661	44,128	72,854	76,480
Total assets	12,693,805		12,693,805		$ 12,425,506
Electric utility
Segment financial information
Total revenues	556,875	495,395	1,075,486	977,447
Income (loss) before income taxes	41,003	58,340	75,725	98,759
Income taxes (benefit)	14,860	21,984	27,618	36,537
Net income	26,143	36,356	48,107	62,222
Preferred stock dividends of subsidiaries	499	499	998	998
Net income for common stock	25,644	35,857	47,109	61,224
Total assets	6,071,155		6,071,155		5,975,428
Bank
Segment financial information
Total revenues	75,329	70,749	148,185	139,589
Income (loss) before income taxes	24,796	20,224	48,956	39,818
Income taxes (benefit)	8,063	6,939	16,410	13,860
Net income	16,733	13,285	32,546	25,958
Preferred stock dividends of subsidiaries	0	0	0	0
Net income for common stock	16,733	13,285	32,546	25,958
Total assets	6,610,877		6,610,877		6,421,357
Other
Segment financial information
Total revenues	77	100	172	168
Income (loss) before income taxes	(6,173)	(7,653)	(13,473)	(16,540)
Income taxes (benefit)	(2,431)	(2,613)	(6,620)	(5,786)
Net income	(3,742)	(5,040)	(6,853)	(10,754)
Preferred stock dividends of subsidiaries	(26)	(26)	(52)	(52)
Net income for common stock	(3,716)	(5,014)	(6,801)	(10,702)
Total assets	11,773		11,773		$ 28,721
Revenues from external customers
Segment financial information
Total revenues	632,281	566,244	1,223,843	1,117,204
Revenues from external customers | Electric utility
Segment financial information
Total revenues	556,836	495,349	1,075,402	977,394
Revenues from external customers | Bank
Segment financial information
Total revenues	75,329	70,749	148,185	139,589
Revenues from external customers | Other
Segment financial information
Total revenues	116	146	256	221
Intersegment revenues (eliminations)
Segment financial information
Total revenues	0	0	0	0
Intersegment revenues (eliminations) | Electric utility
Segment financial information
Total revenues	39	46	84	53
Intersegment revenues (eliminations) | Bank
Segment financial information
Total revenues	0	0	0	0
Intersegment revenues (eliminations) | Other
Segment financial information
Total revenues	$ (39)	$ (46)	$ (84)	$ (53)