Segment financial information (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Jan. 01, 2020	Dec. 31, 2019
Segment financial information
Revenues	$ 641,427			$ 770,882			$ 1,927,558	$ 2,147,982
Income (loss) before income taxes	79,521			78,693			179,447	189,426
Income taxes (benefit)	14,018			14,803			30,691	36,390
Net income	65,503			63,890			148,756	153,036
Preferred stock dividends of subsidiaries	471			471			1,417	1,417
Net income for common stock	65,032	$ 48,887	$ 33,420	63,419	$ 42,512	$ 45,688	147,339	151,619
Total assets	14,542,361						14,542,361		$ 13,745,251	$ 13,745,251
Revenues from external customers
Segment financial information
Revenues	641,427			770,882			1,927,558	2,147,982
Intersegment revenues (eliminations)
Segment financial information
Revenues	0			0			0	0
Electric utility
Segment financial information
Revenues	562,568			688,330			1,694,225	1,900,609
Income (loss) before income taxes	74,398			58,099			157,111	140,775
Income taxes (benefit)	13,835			10,822			29,316	27,800
Net income	60,563			47,277			127,795	112,975
Preferred stock dividends of subsidiaries	498			498			1,496	1,496
Net income for common stock	60,065			46,779			126,299	111,479
Total assets	6,322,304						6,322,304			6,388,682
Electric utility | Revenues from external customers
Segment financial information
Revenues	562,559			688,299			1,694,195	1,900,552
Electric utility | Intersegment revenues (eliminations)
Segment financial information
Revenues	9			31			30	57
Bank
Segment financial information
Revenues	78,644			82,548			233,096	247,287
Income (loss) before income taxes	15,027			29,157			51,330	76,611
Income taxes (benefit)	2,877			6,269			9,405	15,868
Net income	12,150			22,888			41,925	60,743
Preferred stock dividends of subsidiaries	0			0			0	0
Net income for common stock	12,150			22,888			41,925	60,743
Total assets	8,075,745						8,075,745			7,233,017
Bank | Revenues from external customers
Segment financial information
Revenues	78,644			82,548			233,096	247,287
Bank | Intersegment revenues (eliminations)
Segment financial information
Revenues	0			0			0	0
Other
Segment financial information
Revenues	215			4			237	86
Income (loss) before income taxes	(9,904)			(8,563)			(28,994)	(27,960)
Income taxes (benefit)	(2,694)			(2,288)			(8,030)	(7,278)
Net income	(7,210)			(6,275)			(20,964)	(20,682)
Preferred stock dividends of subsidiaries	(27)			(27)			(79)	(79)
Net income for common stock	(7,183)			(6,248)			(20,885)	(20,603)
Total assets	144,312						144,312			$ 123,552
Other | Revenues from external customers
Segment financial information
Revenues	224			35			267	143
Other | Intersegment revenues (eliminations)
Segment financial information
Revenues	$ (9)			$ (31)			$ (30)	$ (57)