Segment financial information (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment financial information
Total revenues	$ 771,535			$ 768,048			$ 2,148,635	$ 2,099,199
Income (loss) before income taxes	78,693			77,233			189,426	190,091
Income taxes (benefit)	14,803			10,862			36,390	36,473
Net income	63,890			66,371			153,036	153,618
Preferred stock dividends of subsidiaries	471			471			1,417	1,417
Net income for common stock	63,419	$ 42,512	$ 45,688	65,900	$ 46,054	$ 40,247	151,619	152,201
Total assets	13,517,970						13,517,970		$ 13,104,051
Revenues from external customers
Segment financial information
Total revenues	771,535			768,048			2,148,635	2,099,199
Intersegment revenues (eliminations)
Segment financial information
Total revenues	0			0			0	0
Electric utility
Segment financial information
Total revenues	688,330			687,409			1,900,609	1,865,962
Income (loss) before income taxes	58,099			57,354			140,775	134,847
Income taxes (benefit)	10,822			7,144			27,800	24,995
Net income	47,277			50,210			112,975	109,852
Preferred stock dividends of subsidiaries	498			498			1,496	1,496
Net income for common stock	46,779			49,712			111,479	108,356
Total assets	6,260,968						6,260,968		5,967,503
Electric utility | Revenues from external customers
Segment financial information
Total revenues	688,299			687,396			1,900,552	1,865,922
Electric utility | Intersegment revenues (eliminations)
Segment financial information
Total revenues	31			13			57	40
Bank
Segment financial information
Total revenues	83,201			80,496			247,940	233,019
Income (loss) before income taxes	29,157			26,831			76,611	77,845
Income taxes (benefit)	6,269			5,610			15,868	17,103
Net income	22,888			21,221			60,743	60,742
Preferred stock dividends of subsidiaries	0			0			0	0
Net income for common stock	22,888			21,221			60,743	60,742
Total assets	7,135,250						7,135,250		7,027,894
Bank | Revenues from external customers
Segment financial information
Total revenues	83,201			80,496			247,940	233,019
Bank | Intersegment revenues (eliminations)
Segment financial information
Total revenues	0			0			0	0
Other
Segment financial information
Total revenues	4			143			86	218
Income (loss) before income taxes	(8,563)			(6,952)			(27,960)	(22,601)
Income taxes (benefit)	(2,288)			(1,892)			(7,278)	(5,625)
Net income	(6,275)			(5,060)			(20,682)	(16,976)
Preferred stock dividends of subsidiaries	(27)			(27)			(79)	(79)
Net income for common stock	(6,248)			(5,033)			(20,603)	(16,897)
Total assets	121,752						121,752		$ 108,654
Other | Revenues from external customers
Segment financial information
Total revenues	35			156			143	258
Other | Intersegment revenues (eliminations)
Segment financial information
Total revenues	$ (31)			$ (13)			$ (57)	$ (40)