Fair value measurements - Changes in Level 3 Liabilities Measured at Fair Value on a Recurring Basis (Details) - Recurring basis - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Jul. 06, 2018
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Derivative liability, beginning balance	$ 430
Fair Value	430
Derivative liability, ending balance	14,498
Level 3
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Derivative liability, beginning balance	0
Fair Value	0
Derivative liability, ending balance	13,000
Contractual liabilities | Level 3
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Derivative liability, beginning balance	0
Fair Value	0	$ (10,117)
Realized and Unrealized Gains/(Losses) in Earnings	(2,883)
Derivative liability, ending balance	$ (13,000)