Segment financial information (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Segment financial information
Revenues	$ 680,257		$ 608,945		$ 1,323,203	$ 1,286,131
Income (loss) before income taxes	82,944		60,230		163,140	99,926
Income taxes (benefit)	18,599		10,870		33,964	16,673
Net income	64,345		49,360		129,176	83,253
Preferred stock dividends of subsidiaries	473		473		946	946
Net income for common stock	63,872	$ 64,358	48,887	$ 33,420	128,230	82,307
Total assets	15,606,729				15,606,729		$ 15,004,007
Revenues from external customers
Segment financial information
Revenues	680,257		608,945		1,323,203	1,286,131
Intersegment revenues (eliminations)
Segment financial information
Revenues	0		0		0	0
Electric utility
Segment financial information
Revenues	601,879		534,215		1,166,743	1,131,657
Income (loss) before income taxes	53,898		53,027		108,988	82,713
Income taxes (benefit)	11,498		10,199		22,731	15,481
Net income	42,400		42,828		86,257	67,232
Preferred stock dividends of subsidiaries	499		499		998	998
Net income for common stock	41,901		42,329		85,259	66,234
Total assets	6,547,974				6,547,974		6,457,373
Electric utility | Revenues from external customers
Segment financial information
Revenues	601,869		534,206		1,166,724	1,131,636
Electric utility | Intersegment revenues (eliminations)
Segment financial information
Revenues	10		9		19	21
Bank
Segment financial information
Revenues	77,260		74,714		154,391	154,452
Income (loss) before income taxes	39,992		17,334		77,094	36,303
Income taxes (benefit)	9,708		3,320		17,254	6,528
Net income	30,284		14,014		59,840	29,775
Preferred stock dividends of subsidiaries	0		0		0	0
Net income for common stock	30,284		14,014		59,840	29,775
Total assets	8,909,904				8,909,904
Bank | Revenues from external customers
Segment financial information
Revenues	77,260		74,714		154,391	154,452
Bank | Intersegment revenues (eliminations)
Segment financial information
Revenues	0		0		0	0
Other
Segment financial information
Revenues	1,118		16		2,069	22
Income (loss) before income taxes	(10,946)		(10,131)		(22,942)	(19,090)
Income taxes (benefit)	(2,607)		(2,649)		(6,021)	(5,336)
Net income	(8,339)		(7,482)		(16,921)	(13,754)
Preferred stock dividends of subsidiaries	(26)		(26)		(52)	(52)
Net income for common stock	(8,313)		(7,456)		(16,869)	(13,702)
Total assets	148,851				148,851		$ 150,101
Other | Revenues from external customers
Segment financial information
Revenues	1,128		25		2,088	43
Other | Intersegment revenues (eliminations)
Segment financial information
Revenues	$ (10)		$ (9)		$ (19)	$ (21)