Fair value of financial instruments - Schedule of Financial Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Fair value measurements on a recurring basis:
Derivative financial instruments	$ 3,720,000	$ 0
Fair Value Measurements on a Recurring Basis
Fair value measurements on a recurring basis:
Deferred consideration		$ 0
Fair Value Measurements on a Recurring Basis | Interest rate swap
Fair value measurements on a recurring basis:
Derivative financial instruments	3,720,000
Fair Value Measurements on a Recurring Basis | Level 1 | Interest rate swap
Fair value measurements on a recurring basis:
Derivative financial instruments	0
Fair Value Measurements on a Recurring Basis | Level 2 | Interest rate swap
Fair value measurements on a recurring basis:
Derivative financial instruments	3,720,000
Fair Value Measurements on a Recurring Basis | Level 3 | Interest rate swap
Fair value measurements on a recurring basis:
Derivative financial instruments	$ 0