Segment financial information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment financial information
Total revenues	$ 661,615	$ 645,874
Income (loss) before income taxes	58,039	53,276
Income taxes (benefit)	11,878	12,556
Net income	46,161	40,720
Preferred stock dividends of subsidiaries	473	473
Net income for common stock	45,688	40,247
Total assets	13,360,105		$ 13,104,051
Revenues from external customers
Segment financial information
Total revenues	661,615	645,874
Intersegment revenues (eliminations)
Segment financial information
Total revenues	0	0
Electric utility
Segment financial information
Total revenues	578,495	570,427
Income (loss) before income taxes	41,859	37,149
Income taxes (benefit)	9,234	9,175
Net income	32,625	27,974
Preferred stock dividends of subsidiaries	499	499
Net income for common stock	32,126	27,475
Total assets	6,170,888		5,967,503
Electric utility | Revenues from external customers
Segment financial information
Total revenues	578,482	570,414
Electric utility | Intersegment revenues (eliminations)
Segment financial information
Total revenues	13	13
Bank
Segment financial information
Total revenues	83,052	75,419
Income (loss) before income taxes	26,162	24,500
Income taxes (benefit)	5,323	5,540
Net income	20,839	18,960
Preferred stock dividends of subsidiaries	0	0
Net income for common stock	20,839	18,960
Total assets	7,062,367		7,027,894
Bank | Revenues from external customers
Segment financial information
Total revenues	83,052	75,419
Bank | Intersegment revenues (eliminations)
Segment financial information
Total revenues	0	0
Other
Segment financial information
Total revenues	68	28
Income (loss) before income taxes	(9,982)	(8,373)
Income taxes (benefit)	(2,679)	(2,159)
Net income	(7,303)	(6,214)
Preferred stock dividends of subsidiaries	(26)	(26)
Net income for common stock	(7,277)	(6,188)
Total assets	126,850		$ 108,654
Other | Revenues from external customers
Segment financial information
Total revenues	81	41
Other | Intersegment revenues (eliminations)
Segment financial information
Total revenues	$ (13)	$ (13)