Fair value measurements - Estimated carrying values, fair values, and levels of the fair value hierarchy of long-term debt (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Estimated fair values of financial instruments (other than derivative financial instruments)
Carrying amounts - Total long-term debt, before deduction of discount on secured term loan and debt issuance costs, excluding obligations under capital leases	$ 773,548	$ 724,208
Level 3
Estimated fair values of financial instruments (other than derivative financial instruments)
Carrying amounts - Total long-term debt, before deduction of discount on secured term loan and debt issuance costs, excluding obligations under capital leases	773,548	724,208
Fair value - Total long-term debt, before deduction of discount on secured term loan and debt issuance costs, excluding obligations under capital leases	$ 768,850	$ 728,994