Income tax expense (benefit) (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current:
Federal	$ (3,502)	$ (15,901)	$ 74,272
State	3,366	3,651	16,192
Current income taxes	(136)	(12,250)	90,464
Deferred:
Federal	50,218	50,755	(24,497)
State	12,098	7,206	(864)
Investment tax credit - net	1,099	1,774	(62)
Deferred income taxes	63,415	59,735	(25,423)
Total income tax expense	$ 63,279	$ 47,485	$ 65,041