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Leases - Additional Information (Detail) - USD ($)
 3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Lessor and Lessee Lease Description [Line Items]    
Operating lease, option to extend, existence true  
Operating lease, option to terminate, existence true  
Short-term lease cost $ 200,000  
Operating lease payments 811,000 $ 1,200,000
Allowance for credit losses for our investment in sales type leases $ 0 $ 0
Minimum [Member]    
Lessor and Lessee Lease Description [Line Items]    
Operating lease, remaining lease terms 1 month  
Operating lease, options to terminate term 3 months  
Maximum [Member]    
Lessor and Lessee Lease Description [Line Items]    
Operating lease, remaining lease terms 6 years  
Operating lease, renewal term 3 years  
Lessor sales type lease arrangement terms for network equipments 5 years