Income tax expense (benefit) (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal	$ 74,272	$ 81,989	$ 85,897
State	16,192	13,190	10,093
Foreign	0	2	30
Current income taxes	90,464	95,181	96,020
Deferred:
Federal	(24,497)	(2,102)	(19,632)
State	(864)	1,184	(5,304)
Investment tax credit - net	(62)	(1,131)	(420)
Deferred income taxes	(25,423)	(2,049)	(25,356)
Total income tax expense	$ 65,041	$ 93,132	$ 70,664