Income tax expense (benefit) (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Current:
Federal	$ (26,858)	$ (7,188)	$ 37,014
State	858	778	10,589
Foreign	(75)	127	4,451
Total current income taxes	(26,075)	(6,283)	52,054
Deferred:
Federal	(1,224)	105,528	62,618
State	(6,323)	13,157	4,147
Investment tax credit - net	44	240	(180)
Total deferred income taxes	(7,503)	118,925	66,585
Change in uncertain tax positions	1,974	(1,048)	3,230
Change in accrued interest	458	(1,320)	661
Total income tax expense (benefit)	$ (31,146)	$ 110,274	$ 122,530