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Amortization Expense (Detail)
$ in Thousands
Dec. 31, 2017
USD ($)
Finite Lived Intangible Assets [Line Items]  
2018 $ 5,648
2019 5,508
2020 5,508
2021 5,295
2022 5,253
Hub [Member]  
Finite Lived Intangible Assets [Line Items]  
2018 4,795
2019 4,655
2020 4,655
2021 4,442
2022 4,400
Mode [Member]  
Finite Lived Intangible Assets [Line Items]  
2018 853
2019 853
2020 853
2021 853
2022 $ 853