Segment financial information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Segment financial information
Total revenues	$ 717,176	$ 867,096	$ 1,978,950	$ 2,449,502
Income (loss) before income taxes	80,660	75,543	189,380	212,595
Income taxes (benefit)	29,516	27,264	70,406	76,302
Net income	51,144	48,279	118,974	136,293
Preferred stock dividends of subsidiaries	471	471	1,417	1,417
Net income for common stock	50,673	47,808	117,557	134,876
Assets	11,573,466		11,573,466		$ 11,185,142
Electric utility
Segment financial information
Total revenues	648,127	803,565	1,779,732	2,262,056
Income (loss) before income taxes	68,894	62,419	164,568	174,711
Income taxes (benefit)	25,390	23,042	60,351	64,686
Net income	43,504	39,377	104,217	110,025
Preferred stock dividends of subsidiaries	498	498	1,496	1,496
Net income for common stock	43,006	38,879	102,721	108,529
Assets	5,683,992		5,683,992		5,590,457
Bank
Segment financial information
Total revenues	69,091	63,536	199,222	187,771
Income (loss) before income taxes	20,802	20,506	61,159	60,994
Income taxes (benefit)	7,351	7,253	21,382	21,806
Net income	13,451	13,253	39,777	39,188
Net income for common stock	13,451	13,253	39,777	39,188
Assets	5,855,497		5,855,497		5,566,222
Other
Segment financial information
Total revenues	(42)	(5)	(4)	(325)
Income (loss) before income taxes	(9,036)	(7,382)	(36,347)	(23,110)
Income taxes (benefit)	(3,225)	(3,031)	(11,327)	(10,190)
Net income	(5,811)	(4,351)	(25,020)	(12,920)
Preferred stock dividends of subsidiaries	(27)	(27)	(79)	(79)
Net income for common stock	(5,784)	(4,324)	(24,941)	(12,841)
Assets	33,977		33,977		$ 28,463
Revenues from external customers
Segment financial information
Total revenues	717,176	867,096	1,978,950	2,449,502
Revenues from external customers | Electric utility
Segment financial information
Total revenues	648,121	803,559	1,779,708	2,262,038
Revenues from external customers | Bank
Segment financial information
Total revenues	69,091	63,536	199,222	187,771
Revenues from external customers | Other
Segment financial information
Total revenues	(36)	1	20	(307)
Intersegment revenues (eliminations) | Electric utility
Segment financial information
Total revenues	6	6	24	18
Intersegment revenues (eliminations) | Other
Segment financial information
Total revenues	$ (6)	$ (6)	$ (24)	$ (18)