Segment financial information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Segment financial information
Total revenues	$ 566,244	$ 623,912	$ 1,117,204	$ 1,261,774
Income (loss) before income taxes	70,911	56,402	122,037	108,720
Income taxes (benefit)	26,310	20,911	44,611	40,890
Net income	44,601	35,491	77,426	67,830
Preferred stock dividends of subsidiaries	473	473	946	946
Net income for common stock	44,128	35,018	76,480	66,884
Total assets	11,979,239		11,979,239		$ 11,782,018
Electric utility
Segment financial information
Total revenues	495,395	558,163	977,447	1,131,605
Income (loss) before income taxes	58,340	52,451	98,759	95,674
Income taxes (benefit)	21,984	19,111	36,537	34,961
Net income	36,356	33,340	62,222	60,713
Preferred stock dividends of subsidiaries	499	499	998	998
Net income for common stock	35,857	32,841	61,224	59,715
Total assets	5,721,853		5,721,853		5,672,210
Bank
Segment financial information
Total revenues	70,749	65,783	139,589	130,131
Income (loss) before income taxes	20,224	19,726	39,818	40,357
Income taxes (benefit)	6,939	6,875	13,860	14,031
Net income	13,285	12,851	25,958	26,326
Preferred stock dividends of subsidiaries	0	0	0	0
Net income for common stock	13,285	12,851	25,958	26,326
Total assets	6,188,090		6,188,090		6,014,755
Other
Segment financial information
Total revenues	100	(34)	168	38
Income (loss) before income taxes	(7,653)	(15,775)	(16,540)	(27,311)
Income taxes (benefit)	(2,613)	(5,075)	(5,786)	(8,102)
Net income	(5,040)	(10,700)	(10,754)	(19,209)
Preferred stock dividends of subsidiaries	(26)	(26)	(52)	(52)
Net income for common stock	(5,014)	(10,674)	(10,702)	(19,157)
Total assets	69,296		69,296		$ 95,053
Revenues from external customers
Segment financial information
Total revenues	566,244	623,912	1,117,204	1,261,774
Revenues from external customers | Electric utility
Segment financial information
Total revenues	495,349	558,156	977,394	1,131,587
Revenues from external customers | Bank
Segment financial information
Total revenues	70,749	65,783	139,589	130,131
Revenues from external customers | Other
Segment financial information
Total revenues	146	(27)	221	56
Intersegment revenues (eliminations)
Segment financial information
Total revenues	0	0	0	0
Intersegment revenues (eliminations) | Electric utility
Segment financial information
Total revenues	46	7	53	18
Intersegment revenues (eliminations) | Bank
Segment financial information
Total revenues	0	0	0	0
Intersegment revenues (eliminations) | Other
Segment financial information
Total revenues	$ (46)	$ (7)	$ (53)	$ (18)