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Commitments and contingencies - Operating lease payments (Details)
$ in Thousands
Dec. 31, 2017
USD ($)
2018  
Disclosure of finance lease and operating lease by lessee [line items]  
Future minimum lease payments $ 90,820
2019  
Disclosure of finance lease and operating lease by lessee [line items]  
Future minimum lease payments 90,035
2020  
Disclosure of finance lease and operating lease by lessee [line items]  
Future minimum lease payments 56,973
2021  
Disclosure of finance lease and operating lease by lessee [line items]  
Future minimum lease payments 38,982
2022  
Disclosure of finance lease and operating lease by lessee [line items]  
Future minimum lease payments 37,858
Thereafter  
Disclosure of finance lease and operating lease by lessee [line items]  
Future minimum lease payments $ 163,822