Basis of presentation and significant accounting policies - Other current liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Accrued interest payable	$ 18,281	$ 18,013
Accrued compensation and benefits	13,317	21,287
Deferred gain on sale of equity method investment	0	20,144
Other accrued liabilities	13,188	16,111
Total other current liabilities	$ 44,786	$ 75,555