Income taxes - Significant Components of our deferred tax assets & liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Accrued postretirement benefit cost	$ 39,417	$ 69,725
Accrued liabilities	3,125	4,679
Share-based compensation	1,319	6,071
Goodwill	1,996	5,539
Net operating losses and tax credits	551,098	739,712
Foreign basis differences	13,875	13,929
Ravenswood retiree legal settlement	3,089	8,683
Other	3,091	5,747
Total deferred tax assets	617,010	854,085
Valuation allowance	(607,813)	(839,082)
Net deferred tax assets	9,197	15,003
Deferred tax liabilities:
Tax over financial statement depreciation	(109,733)	(119,378)
Unremitted foreign earnings	0	(808)
Total deferred tax liabilities	(109,733)	(120,186)
Net deferred tax liability	$ (100,536)	$ (105,183)