Segment financial information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Segment financial information
Total revenues	$ 591,562	$ 550,960
Income (loss) before income taxes	51,582	51,126
Income taxes (benefit)	16,916	18,301
Net income	34,666	32,825
Preferred stock dividends of subsidiaries	473	473
Net income for common stock	34,193	32,352
Total assets	12,543,275		$ 12,425,506
Electric utility
Segment financial information
Total revenues	518,611	482,052
Income (loss) before income taxes	34,722	40,419
Income taxes (benefit)	12,758	14,553
Net income	21,964	25,866
Preferred stock dividends of subsidiaries	499	499
Net income for common stock	21,465	25,367
Total assets	5,969,042		5,975,428
Bank
Segment financial information
Total revenues	72,856	68,840
Income (loss) before income taxes	24,160	19,594
Income taxes (benefit)	8,347	6,921
Net income	15,813	12,673
Preferred stock dividends of subsidiaries	0	0
Net income for common stock	15,813	12,673
Total assets	6,559,646		6,421,357
Other
Segment financial information
Total revenues	95	68
Income (loss) before income taxes	(7,300)	(8,887)
Income taxes (benefit)	(4,189)	(3,173)
Net income	(3,111)	(5,714)
Preferred stock dividends of subsidiaries	(26)	(26)
Net income for common stock	(3,085)	(5,688)
Total assets	14,587		$ 28,721
Revenues from external customers
Segment financial information
Total revenues	591,562	550,960
Revenues from external customers | Electric utility
Segment financial information
Total revenues	518,566	482,045
Revenues from external customers | Bank
Segment financial information
Total revenues	72,856	68,840
Revenues from external customers | Other
Segment financial information
Total revenues	140	75
Intersegment revenues (eliminations)
Segment financial information
Total revenues	0	0
Intersegment revenues (eliminations) | Electric utility
Segment financial information
Total revenues	45	7
Intersegment revenues (eliminations) | Bank
Segment financial information
Total revenues	0	0
Intersegment revenues (eliminations) | Other
Segment financial information
Total revenues	$ (45)	$ (7)