Segment financial information (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Segment financial information
Revenues	$ 756,904			$ 641,427			$ 2,080,107	$ 1,927,558
Income (loss) before income taxes	78,151			79,521			241,291	179,447
Income taxes (benefit)	14,265			14,018			48,229	30,691
Net income	63,886			65,503			193,062	148,756
Preferred stock dividends of subsidiaries	471			471			1,417	1,417
Net income for common stock	63,415	$ 63,872	$ 64,358	65,032	$ 48,887	$ 33,420	191,645	147,339
Total assets	15,760,163						15,760,163		$ 15,004,007
Revenues from external customers
Segment financial information
Revenues	756,904			641,427			2,080,107	1,927,558
Intersegment revenues (eliminations)
Segment financial information
Revenues	0			0			0	0
Electric utility
Segment financial information
Revenues	679,499			562,568			1,846,242	1,694,225
Income (loss) before income taxes	61,175			74,398			170,163	157,111
Income taxes (benefit)	10,335			13,835			33,066	29,316
Net income	50,840			60,563			137,097	127,795
Preferred stock dividends of subsidiaries	498			498			1,496	1,496
Net income for common stock	50,342			60,065			135,601	126,299
Total assets	6,602,908						6,602,908		6,457,373
Electric utility | Revenues from external customers
Segment financial information
Revenues	679,482			562,559			1,846,206	1,694,195
Electric utility | Intersegment revenues (eliminations)
Segment financial information
Revenues	17			9			36	30
Bank
Segment financial information
Revenues	76,208			78,644			230,599	233,096
Income (loss) before income taxes	25,241			15,027			102,335	51,330
Income taxes (benefit)	5,976			2,877			23,230	9,405
Net income	19,265			12,150			79,105	41,925
Preferred stock dividends of subsidiaries	0			0			0	0
Net income for common stock	19,265			12,150			79,105	41,925
Total assets	9,010,419						9,010,419
Bank | Revenues from external customers
Segment financial information
Revenues	76,208			78,644			230,599	233,096
Bank | Intersegment revenues (eliminations)
Segment financial information
Revenues	0			0			0	0
Other
Segment financial information
Revenues	1,197			215			3,266	237
Income (loss) before income taxes	(8,265)			(9,904)			(31,207)	(28,994)
Income taxes (benefit)	(2,046)			(2,694)			(8,067)	(8,030)
Net income	(6,219)			(7,210)			(23,140)	(20,964)
Preferred stock dividends of subsidiaries	(27)			(27)			(79)	(79)
Net income for common stock	(6,192)			(7,183)			(23,061)	(20,885)
Total assets	146,836						146,836		$ 150,101
Other | Revenues from external customers
Segment financial information
Revenues	1,214			224			3,302	267
Other | Intersegment revenues (eliminations)
Segment financial information
Revenues	$ (17)			$ (9)			$ (36)	$ (30)