Reconciliation of income and expenses (Details 5) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (loss) from discontinued operations, net of tax	$ 6,261	$ (202,626)	$ (251,415)	$ (324,605)	$ 27,700	$ 38,482	$ 23,881	$ 25,112	$ (772,385)	$ 115,175	$ 109,879
Discontinued operations, held-for-sale or disposed by sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues									184,853	547,571	536,023
Operating expenses									1,423,037	378,891	364,120
Operating income (loss)									(1,238,184)	168,680	171,903
Other income									2,374	1,163	549
Interest expense									235	110	114
Income (loss) from discontinued operations before income taxes									(1,236,045)	169,733	172,338
Income taxes									(463,660)	54,558	62,459
Income (loss) from discontinued operations, net of tax									$ (772,385)	$ 115,175	$ 109,879