Segment financial information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015		Jun. 30, 2014		Dec. 31, 2014
Segment financial information
Total revenues	$ 623,912	$ 798,657	$ 1,261,774		$ 1,582,406
Income (loss) before income taxes	56,402	65,071	108,720		137,052
Income taxes (benefit)	20,911	23,317	40,890		49,038
Net income	35,491	41,754	67,830	[1]	88,014	[1]
Preferred stock dividends of subsidiaries	473	473	946		946
Net income for common stock	35,018	41,281	66,884		87,068
Assets	11,439,631		11,439,631				$ 11,185,142
Electric utility
Segment financial information
Total revenues	558,163	738,429	1,131,605		1,458,491
Income (loss) before income taxes	52,451	55,126	95,674		112,292
Income taxes (benefit)	19,111	20,397	34,961		41,644
Net income	33,340	34,729	60,713		70,648
Preferred stock dividends of subsidiaries	499	499	998		998
Net income for common stock	32,841	34,230	59,715		69,650
Assets	5,634,307		5,634,307				5,590,457
Bank
Segment financial information
Total revenues	65,783	60,616	130,131		124,235
Income (loss) before income taxes	19,726	17,956	40,357		40,488
Income taxes (benefit)	6,875	6,420	14,031		14,553
Net income	12,851	11,536	26,326		25,935
Net income for common stock	12,851	11,536	26,326		25,935
Assets	5,777,098		5,777,098				5,566,222
Other
Segment financial information
Total revenues	(34)	(388)	38		(320)
Income (loss) before income taxes	(15,775)	(8,011)	(27,311)		(15,728)
Income taxes (benefit)	(5,075)	(3,500)	(8,102)		(7,159)
Net income	(10,700)	(4,511)	(19,209)		(8,569)
Preferred stock dividends of subsidiaries	(26)	(26)	(52)		(52)
Net income for common stock	(10,674)	(4,485)	(19,157)		(8,517)
Assets	28,226		28,226				$ 28,463
Revenues from external customers
Segment financial information
Total revenues	623,912	798,657	1,261,774		1,582,406
Revenues from external customers | Electric utility
Segment financial information
Total revenues	558,156	738,423	1,131,587		1,458,479
Revenues from external customers | Bank
Segment financial information
Total revenues	65,783	60,616	130,131		124,235
Revenues from external customers | Other
Segment financial information
Total revenues	(27)	(382)	56		(308)
Intersegment revenues (eliminations) | Electric utility
Segment financial information
Total revenues	7	6	18		12
Intersegment revenues (eliminations) | Other
Segment financial information
Total revenues	$ (7)	$ (6)	$ (18)		$ (12)

[1] As restated - See Note 1, “Basis of presentation - Restatement of previously issued financial statements.”