Income taxes (Additional Information) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Line Items]
Operating loss carryforwards not subject to expiration	$ 2,351	$ 2,383
Federal
Income Tax Disclosure [Line Items]
Operating loss carryforwards	1,230,419
Operating loss carryforwards subject to expiration	1,011,368		$ 1,011,368
Operating loss carryforwards not subject to expiration	219,051
Percentage depletion carryforwards, not subject to expiration	11,194
Operating loss carryforwards, subject to limitation			760,067
Operating loss carryforwards, not subject to limitation			$ 251,301
State
Income Tax Disclosure [Line Items]
Operating loss carryforwards	$ 1,498,363