Income tax expense (benefit) (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current:
Federal	$ 65,006	$ (3,502)	$ (15,901)
State	21,234	3,366	3,651
Foreign	151	0	0
Current income taxes	86,391	(136)	(12,250)
Deferred:
Federal	(3,735)	50,218	50,755
State	(625)	12,098	7,206
Investment tax credit - net	2,559	1,099	1,774
Deferred income taxes	(1,801)	63,415	59,735
Total income tax expense	$ 84,590	$ 63,279	$ 47,485