Income taxes - Operating losses (Details) - Federal $ in Thousands	Dec. 31, 2020 USD ($)
Operating Loss Carryforwards [Line Items]
Total expiring federal net operating loss carryforwards	$ 1,737,098
Non-expiring federal net operating loss carryforwards	369,536
Total federal net operating loss carryforwards	2,106,634
2026
Operating Loss Carryforwards [Line Items]
Total expiring federal net operating loss carryforwards	2,741
2027
Operating Loss Carryforwards [Line Items]
Total expiring federal net operating loss carryforwards	38,651
2028
Operating Loss Carryforwards [Line Items]
Total expiring federal net operating loss carryforwards	228,661
2029
Operating Loss Carryforwards [Line Items]
Total expiring federal net operating loss carryforwards	101,932
2030
Operating Loss Carryforwards [Line Items]
Total expiring federal net operating loss carryforwards	80,963
Thereafter
Operating Loss Carryforwards [Line Items]
Total expiring federal net operating loss carryforwards	$ 1,284,150