Segment financial information (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Jan. 01, 2020	Dec. 31, 2019
Segment financial information
Total revenues	$ 608,945		$ 715,485		$ 1,286,131	$ 1,377,100
Income (loss) before income taxes	60,230		52,694		99,926	110,733
Income taxes (benefit)	10,870		9,709		16,673	21,587
Net income	49,360		42,985		83,253	89,146
Preferred stock dividends of subsidiaries	473		473		946	946
Net income for common stock	48,887	$ 33,420	42,512	$ 45,688	82,307	88,200
Total assets	14,480,700				14,480,700		$ 13,745,251	$ 13,745,251
Revenues from external customers
Segment financial information
Total revenues	608,945		715,485		1,286,131	1,377,100
Intersegment revenues (eliminations)
Segment financial information
Total revenues	0		0		0	0
Electric utility
Segment financial information
Total revenues	534,215		633,784		1,131,657	1,212,279
Income (loss) before income taxes	53,027		40,817		82,713	82,676
Income taxes (benefit)	10,199		7,744		15,481	16,978
Net income	42,828		33,073		67,232	65,698
Preferred stock dividends of subsidiaries	499		499		998	998
Net income for common stock	42,329		32,574		66,234	64,700
Total assets	6,332,007				6,332,007			6,388,682
Electric utility | Revenues from external customers
Segment financial information
Total revenues	534,206		633,771		1,131,636	1,212,253
Electric utility | Intersegment revenues (eliminations)
Segment financial information
Total revenues	9		13		21	26
Bank
Segment financial information
Total revenues	74,714		81,687		154,452	164,739
Income (loss) before income taxes	17,334		21,292		36,303	47,454
Income taxes (benefit)	3,320		4,276		6,528	9,599
Net income	14,014		17,016		29,775	37,855
Preferred stock dividends of subsidiaries	0		0		0	0
Net income for common stock	14,014		17,016		29,775	37,855
Total assets	8,019,665				8,019,665			7,233,017
Bank | Revenues from external customers
Segment financial information
Total revenues	74,714		81,687		154,452	164,739
Bank | Intersegment revenues (eliminations)
Segment financial information
Total revenues	0		0		0	0
Other
Segment financial information
Total revenues	16		14		22	82
Income (loss) before income taxes	(10,131)		(9,415)		(19,090)	(19,397)
Income taxes (benefit)	(2,649)		(2,311)		(5,336)	(4,990)
Net income	(7,482)		(7,104)		(13,754)	(14,407)
Preferred stock dividends of subsidiaries	(26)		(26)		(52)	(52)
Net income for common stock	(7,456)		(7,078)		(13,702)	(14,355)
Total assets	129,028				129,028			$ 123,552
Other | Revenues from external customers
Segment financial information
Total revenues	25		27		43	108
Other | Intersegment revenues (eliminations)
Segment financial information
Total revenues	$ (9)		$ (13)		$ (21)	$ (26)