Segment financial information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment financial information
Total revenues	$ 768,048	$ 673,185	$ 2,099,199	$ 1,897,028
Income (loss) before income taxes	77,233	95,139	190,091	206,347
Income taxes (benefit)	10,862	34,595	36,473	72,003
Net income	66,371	60,544	153,618	134,344
Preferred stock dividends of subsidiaries	471	471	1,417	1,417
Net income for common stock	65,900	60,073	152,201	132,927
Total assets	12,911,554		12,911,554		$ 12,534,160
Revenues from external customers
Segment financial information
Total revenues	768,048	673,185	2,099,199	1,897,028
Intersegment revenues (eliminations)
Segment financial information
Total revenues	0	0	0	0
Electric utility
Segment financial information
Total revenues	687,409	598,769	1,865,962	1,674,255
Income (loss) before income taxes	57,354	74,990	134,847	150,715
Income taxes (benefit)	7,144	27,005	24,995	54,623
Net income	50,210	47,985	109,852	96,092
Preferred stock dividends of subsidiaries	498	498	1,496	1,496
Net income for common stock	49,712	47,487	108,356	94,596
Total assets	5,882,127		5,882,127		5,630,613
Electric utility | Revenues from external customers
Segment financial information
Total revenues	687,396	598,756	1,865,922	1,674,158
Electric utility | Intersegment revenues (eliminations)
Segment financial information
Total revenues	13	13	40	97
Bank
Segment financial information
Total revenues	80,496	74,289	233,019	222,474
Income (loss) before income taxes	26,831	26,764	77,845	75,720
Income taxes (benefit)	5,610	9,172	17,103	25,582
Net income	21,221	17,592	60,742	50,138
Preferred stock dividends of subsidiaries	0	0	0	0
Net income for common stock	21,221	17,592	60,742	50,138
Total assets	6,929,456		6,929,456		6,798,659
Bank | Revenues from external customers
Segment financial information
Total revenues	80,496	74,289	233,019	222,474
Bank | Intersegment revenues (eliminations)
Segment financial information
Total revenues	0	0	0	0
Other
Segment financial information
Total revenues	143	127	218	299
Income (loss) before income taxes	(6,952)	(6,615)	(22,601)	(20,088)
Income taxes (benefit)	(1,892)	(1,582)	(5,625)	(8,202)
Net income	(5,060)	(5,033)	(16,976)	(11,886)
Preferred stock dividends of subsidiaries	(27)	(27)	(79)	(79)
Net income for common stock	(5,033)	(5,006)	(16,897)	(11,807)
Total assets	99,971		99,971		$ 104,888
Other | Revenues from external customers
Segment financial information
Total revenues	156	140	258	396
Other | Intersegment revenues (eliminations)
Segment financial information
Total revenues	$ (13)	$ (13)	$ (40)	$ (97)