Income tax expense (benefit) (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
Federal	$ (15,901)	$ 74,272	$ 81,989
State	3,651	16,192	13,190
Foreign	0	0	2
Current income taxes	(12,250)	90,464	95,181
Deferred:
Federal	50,755	(24,497)	(2,102)
State	7,206	(864)	1,184
Investment tax credit - net	1,774	(62)	(1,131)
Deferred income taxes	59,735	(25,423)	(2,049)
Total income tax expense	$ 47,485	$ 65,041	$ 93,132