Segment financial information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Jan. 01, 2020	Dec. 31, 2019
Segment financial information
Total revenues	$ 677,186	$ 661,615
Income (loss) before income taxes	39,696	58,039
Income taxes (benefit)	5,803	11,878
Net income	33,893	46,161
Preferred stock dividends of subsidiaries	473	473
Net income for common stock	33,420	45,688
Total assets	13,849,018		$ 13,745,251	$ 13,745,251
Revenues from external customers
Segment financial information
Total revenues	677,186	661,615
Intersegment revenues (eliminations)
Segment financial information
Total revenues	0	0
Electric utility
Segment financial information
Total revenues	597,442	578,495
Income (loss) before income taxes	29,686	41,859
Income taxes (benefit)	5,282	9,234
Net income	24,404	32,625
Preferred stock dividends of subsidiaries	499	499
Net income for common stock	23,905	32,126
Total assets	6,335,928			6,388,682
Electric utility | Revenues from external customers
Segment financial information
Total revenues	597,430	578,482
Electric utility | Intersegment revenues (eliminations)
Segment financial information
Total revenues	12	13
Bank
Segment financial information
Total revenues	79,738	83,052
Income (loss) before income taxes	18,969	26,162
Income taxes (benefit)	3,208	5,323
Net income	15,761	20,839
Preferred stock dividends of subsidiaries	0	0
Net income for common stock	15,761	20,839
Total assets	7,385,745			7,233,017
Bank | Revenues from external customers
Segment financial information
Total revenues	79,738	83,052
Bank | Intersegment revenues (eliminations)
Segment financial information
Total revenues	0	0
Other
Segment financial information
Total revenues	6	68
Income (loss) before income taxes	(8,959)	(9,982)
Income taxes (benefit)	(2,687)	(2,679)
Net income	(6,272)	(7,303)
Preferred stock dividends of subsidiaries	(26)	(26)
Net income for common stock	(6,246)	(7,277)
Total assets	127,345			$ 123,552
Other | Revenues from external customers
Segment financial information
Total revenues	18	81
Other | Intersegment revenues (eliminations)
Segment financial information
Total revenues	$ (12)	$ (13)