Income tax expense (benefit) (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current:
Federal	$ 59,483	$ 4,403	$ 41,624
State	5,789	(166)	4,148
Foreign	30	0	(29)
Current income taxes	65,302	4,237	45,743
Deferred:
Federal	3,199	55,514	29,616
State	(2,478)	2,467	(859)
Investment tax credit - net	(420)	1,009	(206)
Deferred income taxes	301	58,990	28,551
Total income tax expense	$ 65,603	$ 63,227	$ 74,294