Leases (Tables)

12 Months Ended

Dec. 31, 2017

Leases [Abstract]

Future minimum lease payments

The future minimum lease payments relating to operating leases are summarized as follows (in thousands of dollars):

2018	52,112
2019	39,996
2020	27,184
2021	18,147
2022	11,851
2023 and after	17,843
	167,133