Segment financial information (Details) $ in Thousands			3 Months Ended								12 Months Ended
	Feb. 02, 2018 MWh project MW	Nov. 24, 2017 MW	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) MW	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment financial information
Revenues											$ 2,614,831		$ 2,380,654
Revenues			$ 761,650	$ 768,048	$ 685,277	$ 645,874	$ 658,597	$ 673,185	$ 632,281	$ 591,562	2,860,849	$ 2,555,625	2,380,654
Depreciation and amortization											255,629	221,998	204,746
Interest expense, net											104,216	91,128	88,558
Income (loss) before income taxes											254,461	276,580	373,841
Income taxes (benefit)											50,797	109,393	123,695
Net income (loss)			50,046	66,371	46,527	40,720	32,843	60,544	39,134	34,666	203,664	167,187	250,146
Preferred stock dividends of subsidiaries											1,890	1,890	1,890
Net income for common stock			49,573	$ 65,900	$ 46,054	$ 40,247	32,370	$ 60,073	$ 38,661	$ 34,193	201,774	165,297	248,256
Capital expenditures											537,369	495,187	330,043
Assets			13,104,051				12,534,160				13,104,051	12,534,160	11,881,981
Electric utility
Segment financial information
Revenues											2,567,531		2,094,368
Revenues											2,546,525	2,257,566	2,094,368
Depreciation and amortization											230,228	201,282	193,996
Interest expense, net											73,348	69,637	66,824
Income (loss) before income taxes											180,426	205,145	229,113
Income taxes (benefit)											34,778	83,199	84,801
Net income (loss)											145,648	121,946	144,312
Preferred stock dividends of subsidiaries											1,995	1,995	1,995
Net income for common stock											143,653	119,951	142,317
Capital expenditures											445,863	441,598	320,437
Assets			5,967,503				5,630,613				5,967,503	5,630,613	5,431,903
Bank
Segment financial information
Revenues											47,300		285,924
Revenues											314,275	297,640	285,924
Depreciation and amortization											21,443	19,416	9,813
Interest expense, net											15,539	12,156	12,755
Income (loss) before income taxes											106,578	98,716	87,352
Income taxes (benefit)											24,069	31,719	30,073
Net income (loss)											82,509	66,997	57,279
Preferred stock dividends of subsidiaries											0	0	0
Net income for common stock											82,509	66,997	57,279
Capital expenditures											72,666	53,272	9,394
Assets			7,027,894				6,798,659				7,027,894	6,798,659	6,421,357
Other
Segment financial information
Revenues											0		362
Revenues											49	419	362
Depreciation and amortization											3,958	1,300	937
Interest expense, net											15,329	9,335	8,979
Income (loss) before income taxes											(32,543)	(27,281)	57,376
Income taxes (benefit)											(8,050)	(5,525)	8,821
Net income (loss)											(24,493)	(21,756)	48,555
Preferred stock dividends of subsidiaries											(105)	(105)	(105)
Net income for common stock											(24,388)	(21,651)	48,660
Capital expenditures											18,840	317	212
Assets			$ 108,654				$ 104,888				108,654	104,888	28,721
Operating Segments
Segment financial information
Revenues											2,860,849	2,555,625	2,380,654
Operating Segments | Electric utility
Segment financial information
Revenues											2,546,472	2,257,455	2,094,224
Operating Segments | Bank
Segment financial information
Revenues											314,275	297,640	285,924
Operating Segments | Other
Segment financial information
Revenues											102	530	506
Intersegment eliminations
Segment financial information
Revenues											0	0	0
Intersegment eliminations | Electric utility
Segment financial information
Revenues											53	111	144
Intersegment eliminations | Bank
Segment financial information
Revenues											0	0	0
Intersegment eliminations | Other
Segment financial information
Revenues											$ (53)	$ (111)	$ (144)
Hamakua Energy Partners, L.P. (HEP)
Segment financial information
Power produced by power plants (in megawatts) | MW		60									60
Mauo, LLC
Segment financial information
Power produced by power plants (in megawatts) | MW	8.6
Power storage capacity (in megawatt hours) | MWh	42.3
Power purchase agreement term	15 years
Power purchase agreement extension term	5 years
Number of project sites expected to be placed in service during 2019 and 2020 | project	5