Income tax expense (benefit) (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current:
Federal	$ 81,989	$ 85,897	$ 8,837
State	13,190	10,093	622
Foreign	2	30	0
Current income taxes	95,181	96,020	9,459
Deferred:
Federal	(2,102)	(19,632)	52,041
State	1,184	(5,304)	1,913
Investment tax credit - net	(1,131)	(420)	1,009
Deferred income taxes	(2,049)	(25,356)	54,963
Total income tax expense	$ 93,132	$ 70,664	$ 64,422