Segment financial information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment financial information
Total revenues	$ 645,874	$ 591,562
Income (loss) before income taxes	53,276	51,582
Income taxes (benefit)	12,556	16,916
Net income	40,720	34,666
Preferred stock dividends of subsidiaries	473	473
Net income for common stock	40,247	34,193
Total assets	12,702,718		$ 12,534,160
Revenues from external customers
Segment financial information
Total revenues	645,874	591,562
Intersegment revenues (eliminations)
Segment financial information
Total revenues	0	0
Electric utility
Segment financial information
Total revenues	570,427	518,611
Income (loss) before income taxes	37,149	34,722
Income taxes (benefit)	9,175	12,758
Net income	27,974	21,964
Preferred stock dividends of subsidiaries	499	499
Net income for common stock	27,475	21,465
Total assets	5,710,569		5,630,613
Electric utility | Revenues from external customers
Segment financial information
Total revenues	570,414	518,566
Electric utility | Intersegment revenues (eliminations)
Segment financial information
Total revenues	13	45
Bank
Segment financial information
Total revenues	75,419	72,856
Income (loss) before income taxes	24,500	24,160
Income taxes (benefit)	5,540	8,347
Net income	18,960	15,813
Preferred stock dividends of subsidiaries	0	0
Net income for common stock	18,960	15,813
Total assets	6,889,445		6,798,659
Bank | Revenues from external customers
Segment financial information
Total revenues	75,419	72,856
Bank | Intersegment revenues (eliminations)
Segment financial information
Total revenues	0	0
Other
Segment financial information
Total revenues	28	95
Income (loss) before income taxes	(8,373)	(7,300)
Income taxes (benefit)	(2,159)	(4,189)
Net income	(6,214)	(3,111)
Preferred stock dividends of subsidiaries	(26)	(26)
Net income for common stock	(6,188)	(3,085)
Total assets	102,704		$ 104,888
Other | Revenues from external customers
Segment financial information
Total revenues	41	140
Other | Intersegment revenues (eliminations)
Segment financial information
Total revenues	$ (13)	$ (45)