Segment financial information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Segment financial information
Total revenues	$ 646,055	$ 717,176	$ 1,763,259	$ 1,978,950
Income (loss) before income taxes	179,205	80,660	301,242	189,380
Income taxes (benefit)	51,592	29,516	96,203	70,406
Net income	127,613	51,144	205,039	118,974
Preferred stock dividends of subsidiaries	471	471	1,417	1,417
Net income for common stock	127,142	50,673	203,622	117,557
Total assets	12,198,985		12,198,985		$ 11,782,018
Electric utility
Segment financial information
Total revenues	572,253	648,127	1,549,700	1,779,732
Income (loss) before income taxes	75,617	68,894	174,376	164,568
Income taxes (benefit)	28,145	25,390	64,682	60,351
Net income	47,472	43,504	109,694	104,217
Preferred stock dividends of subsidiaries	498	498	1,496	1,496
Net income for common stock	46,974	43,006	108,198	102,721
Total assets	5,800,826		5,800,826		5,672,210
Bank
Segment financial information
Total revenues	73,708	69,091	213,297	199,222
Income (loss) before income taxes	22,727	20,802	62,545	61,159
Income taxes (benefit)	7,623	7,351	21,483	21,382
Net income	15,104	13,451	41,062	39,777
Preferred stock dividends of subsidiaries	0	0	0	0
Net income for common stock	15,104	13,451	41,062	39,777
Total assets	6,336,670		6,336,670		6,014,755
Other
Segment financial information
Total revenues	94	(42)	262	(4)
Income (loss) before income taxes	80,861	(9,036)	64,321	(36,347)
Income taxes (benefit)	15,824	(3,225)	10,038	(11,327)
Net income	65,037	(5,811)	54,283	(25,020)
Preferred stock dividends of subsidiaries	(27)	(27)	(79)	(79)
Net income for common stock	65,064	(5,784)	54,362	(24,941)
Total assets	61,489		61,489		$ 95,053
Revenues from external customers
Segment financial information
Total revenues	646,055	717,176	1,763,259	1,978,950
Revenues from external customers | Electric utility
Segment financial information
Total revenues	572,208	648,121	1,549,602	1,779,708
Revenues from external customers | Bank
Segment financial information
Total revenues	73,708	69,091	213,297	199,222
Revenues from external customers | Other
Segment financial information
Total revenues	139	(36)	360	20
Intersegment revenues (eliminations)
Segment financial information
Total revenues	0	0	0	0
Intersegment revenues (eliminations) | Electric utility
Segment financial information
Total revenues	45	6	98	24
Intersegment revenues (eliminations) | Bank
Segment financial information
Total revenues	0	0	0	0
Intersegment revenues (eliminations) | Other
Segment financial information
Total revenues	$ (45)	$ (6)	$ (98)	$ (24)