Segment financial information (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment financial information
Total revenues	$ 715,485		$ 685,277		$ 1,377,100	$ 1,331,151
Income (loss) before income taxes	52,694		59,582		110,733	112,858
Income taxes (benefit)	9,709		13,055		21,587	25,611
Net income	42,985		46,527		89,146	87,247
Preferred stock dividends of subsidiaries	473		473		946	946
Net income for common stock	42,512	$ 45,688	46,054	$ 40,247	88,200	86,301
Total assets	13,537,766				13,537,766		$ 13,104,051
Revenues from external customers
Segment financial information
Total revenues	715,485		685,277		1,377,100	1,331,151
Intersegment revenues (eliminations)
Segment financial information
Total revenues	0		0		0	0
Electric utility
Segment financial information
Total revenues	633,784		608,126		1,212,279	1,178,553
Income (loss) before income taxes	40,817		40,344		82,676	77,493
Income taxes (benefit)	7,744		8,676		16,978	17,851
Net income	33,073		31,668		65,698	59,642
Preferred stock dividends of subsidiaries	499		499		998	998
Net income for common stock	32,574		31,169		64,700	58,644
Total assets							5,967,503
Electric utility | Revenues from external customers
Segment financial information
Total revenues	633,771		608,112		1,212,253	1,178,526
Electric utility | Intersegment revenues (eliminations)
Segment financial information
Total revenues	13		14		26	27
Bank
Segment financial information
Total revenues	81,687		77,104		164,739	152,523
Income (loss) before income taxes	21,292		26,514		47,454	51,014
Income taxes (benefit)	4,276		5,953		9,599	11,493
Net income	17,016		20,561		37,855	39,521
Preferred stock dividends of subsidiaries	0		0		0	0
Net income for common stock	17,016		20,561		37,855	39,521
Total assets	7,164,054				7,164,054		7,027,894
Bank | Revenues from external customers
Segment financial information
Total revenues	81,687		77,104		164,739	152,523
Bank | Intersegment revenues (eliminations)
Segment financial information
Total revenues	0		0		0	0
Other
Segment financial information
Total revenues	14		47		82	75
Income (loss) before income taxes	(9,415)		(7,276)		(19,397)	(15,649)
Income taxes (benefit)	(2,311)		(1,574)		(4,990)	(3,733)
Net income	(7,104)		(5,702)		(14,407)	(11,916)
Preferred stock dividends of subsidiaries	(26)		(26)		(52)	(52)
Net income for common stock	(7,078)		(5,676)		(14,355)	(11,864)
Total assets	119,866				119,866		$ 108,654
Other | Revenues from external customers
Segment financial information
Total revenues	27		61		108	102
Other | Intersegment revenues (eliminations)
Segment financial information
Total revenues	$ (13)		$ (14)		$ (26)	$ (27)