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, 2017	Dec. 31, 2016
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	$ 724,208	$ 602,083
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	724,208	602,083
Fair value - Total long-term debt, before deduction of discount on secured term loan and debt issuance costs, excluding obligations under capital leases	$ 728,994	$ 626,613