Segment financial information (Details) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) MW	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Segment financial information
Total revenues		$ 673,185	$ 646,055	$ 1,897,028	$ 1,763,259
Income (loss) before income taxes		95,139	179,205	206,347	301,242
Income taxes (benefit)		34,595	51,592	72,003	96,203
Net income		60,544	127,613	134,344	205,039
Preferred stock dividends of subsidiaries		471	471	1,417	1,417
Net income for common stock		60,073	127,142	132,927	203,622
Total assets	$ 12,742,850	12,742,850		12,742,850		$ 12,425,506
Hamakua Energy Partners, L.P. (HEP)
Segment financial information
Power produced by power plant (in megawatts) | MW	60
Revenues from external customers
Segment financial information
Total revenues		673,185	646,055	1,897,028	1,763,259
Intersegment revenues (eliminations)
Segment financial information
Total revenues		0	0	0	0
Electric utility
Segment financial information
Total revenues		598,769	572,253	1,674,255	1,549,700
Income (loss) before income taxes		74,990	75,617	150,715	174,376
Income taxes (benefit)		27,005	28,145	54,623	64,682
Net income		47,985	47,472	96,092	109,694
Preferred stock dividends of subsidiaries		498	498	1,496	1,496
Net income for common stock		47,487	46,974	94,596	108,198
Total assets	$ 6,107,501	6,107,501		6,107,501		5,975,428
Electric utility | Revenues from external customers
Segment financial information
Total revenues		598,756	572,208	1,674,158	1,549,602
Electric utility | Intersegment revenues (eliminations)
Segment financial information
Total revenues		13	45	97	98
Bank
Segment financial information
Total revenues		74,289	73,708	222,474	213,297
Income (loss) before income taxes		26,764	22,727	75,720	62,545
Income taxes (benefit)		9,172	7,623	25,582	21,483
Net income		17,592	15,104	50,138	41,062
Preferred stock dividends of subsidiaries		0	0	0	0
Net income for common stock		17,592	15,104	50,138	41,062
Total assets	6,618,907	6,618,907		6,618,907		6,421,357
Bank | Revenues from external customers
Segment financial information
Total revenues		74,289	73,708	222,474	213,297
Bank | Intersegment revenues (eliminations)
Segment financial information
Total revenues		0	0	0	0
Other
Segment financial information
Total revenues		127	94	299	262
Income (loss) before income taxes		(6,615)	80,861	(20,088)	64,321
Income taxes (benefit)		(1,582)	15,824	(8,202)	10,038
Net income		(5,033)	65,037	(11,886)	54,283
Preferred stock dividends of subsidiaries		(27)	(27)	(79)	(79)
Net income for common stock		(5,006)	65,064	(11,807)	54,362
Total assets	$ 16,442	16,442		16,442		$ 28,721
Other | Revenues from external customers
Segment financial information
Total revenues		140	139	396	360
Other | Intersegment revenues (eliminations)
Segment financial information
Total revenues		$ (13)	$ (45)	$ (97)	$ (98)