Income tax expense (benefit) (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Current:
Federal	$ 45,518	$ (26,858)	$ (7,188)
State	4,311	858	778
Foreign	(29)	(75)	127
Current income taxes	49,800	(26,075)	(6,283)
Deferred:
Federal	78,953	(1,224)	105,528
State	8,031	(6,323)	13,157
Investment tax credit - net	(206)	44	240
Deferred income taxes	86,778	(7,503)	118,925
Change in uncertain tax positions	0	1,974	(1,048)
Change in accrued interest	158	458	(1,320)
Total income tax expense (benefit)	$ 136,736	$ (31,146)	$ 110,274