Fair value of financial instruments - Schedule of Financial Liabilities Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Carrying Value
Financial liabilities not recorded at fair value:
Debt	$ 989,387	$ 898,215
Carrying Value | Term Loan
Financial liabilities not recorded at fair value:
Debt	989,387	898,215
Carrying Value | Revolving Credit Facility
Financial liabilities not recorded at fair value:
Debt	0	0
Fair Value | Level 1
Financial liabilities not recorded at fair value:
Debt	0	0
Fair Value | Level 1 | Term Loan
Financial liabilities not recorded at fair value:
Debt	0	0
Fair Value | Level 1 | Revolving Credit Facility
Financial liabilities not recorded at fair value:
Debt	0	0
Fair Value | Level 2
Financial liabilities not recorded at fair value:
Debt	0	0
Fair Value | Level 2 | Term Loan
Financial liabilities not recorded at fair value:
Debt	0	0
Fair Value | Level 2 | Revolving Credit Facility
Financial liabilities not recorded at fair value:
Debt	0	0
Fair Value | Level 3
Financial liabilities not recorded at fair value:
Debt	927,025	916,369
Fair Value | Level 3 | Term Loan
Financial liabilities not recorded at fair value:
Debt	927,025	916,369
Fair Value | Level 3 | Revolving Credit Facility
Financial liabilities not recorded at fair value:
Debt	$ 0	$ 0