Segment financial information (Details) $ in Thousands			3 Months Ended								12 Months Ended
	Feb. 02, 2018 MWh project MW	Nov. 24, 2017 MW	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) MW	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment financial information
Revenues			$ 725,966	$ 771,535	$ 715,485	$ 661,615	$ 761,650	$ 768,048	$ 685,277	$ 645,874	$ 2,874,601	$ 2,860,849	$ 2,555,625
Depreciation and amortization											278,113	255,629	221,998
Interest expense, net											109,339	104,216	91,128
Income (loss) before income taxes											271,409	254,461	276,580
Income taxes (benefit)											51,637	50,797	109,393
Net income (loss)			66,736	63,890	42,985	46,161	50,046	66,371	46,527	40,720	219,772	203,664	167,187
Preferred stock dividends of subsidiaries											1,890	1,890	1,890
Net income for common stock			66,263	$ 63,419	$ 42,512	$ 45,688	49,573	$ 65,900	$ 46,054	$ 40,247	217,882	201,774	165,297
Capital expenditures											457,520	537,369	495,187
Assets			13,745,251				13,104,051				13,745,251	13,104,051	12,534,160
Electric utility
Segment financial information
Revenues											2,545,942	2,546,525	2,257,566
Depreciation and amortization											245,362	230,228	201,282
Interest expense, net											70,842	73,348	69,637
Income (loss) before income taxes											197,140	180,426	205,145
Income taxes (benefit)											38,305	34,778	83,199
Net income (loss)											158,835	145,648	121,946
Preferred stock dividends of subsidiaries											1,995	1,995	1,995
Net income for common stock											156,840	143,653	119,951
Capital expenditures											419,898	415,264	376,865
Assets			6,388,682				5,967,503				6,388,682	5,967,503	5,630,613
Bank
Segment financial information
Revenues											328,570	314,275	297,640
Depreciation and amortization											28,675	21,443	19,416
Interest expense, net											18,440	15,539	12,156
Income (loss) before income taxes											112,034	106,578	98,716
Income taxes (benefit)											23,061	24,069	31,719
Net income (loss)											88,973	82,509	66,997
Preferred stock dividends of subsidiaries											0	0	0
Net income for common stock											88,973	82,509	66,997
Capital expenditures											24,175	72,666	53,272
Assets			7,233,017				7,027,894				7,233,017	7,027,894	6,798,659
Other
Segment financial information
Revenues											89	49	419
Depreciation and amortization											4,076	3,958	1,300
Interest expense, net											20,057	15,329	9,335
Income (loss) before income taxes											(37,765)	(32,543)	(27,281)
Income taxes (benefit)											(9,729)	(8,050)	(5,525)
Net income (loss)											(28,036)	(24,493)	(21,756)
Preferred stock dividends of subsidiaries											(105)	(105)	(105)
Net income for common stock											(27,931)	(24,388)	(21,651)
Capital expenditures											13,447	18,840	317
Assets			$ 123,552				$ 108,654				123,552	108,654	104,888
Operating Segments | Electric utility
Segment financial information
Revenues											2,545,865	2,546,472	2,257,455
Operating Segments | Bank
Segment financial information
Revenues											328,570	314,275	297,640
Operating Segments | Other
Segment financial information
Revenues											166	102	530
Intersegment eliminations | Electric utility
Segment financial information
Revenues											77	53	111
Intersegment eliminations | Bank
Segment financial information
Revenues											0	0	0
Intersegment eliminations | Other
Segment financial information
Revenues											$ (77)	$ (53)	$ (111)
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