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Property and Equipment - Additional Information (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Property, Plant and Equipment [Line Items]      
Property and equipment, net $ 53,359 $ 57,436  
Accumulated depreciation 101,380 100,354  
Property and equipment, gross 154,739 157,790  
Depreciation and amortization 8,400 10,600 $ 14,000
Aircraft And Aircraft-Related Equipment      
Property, Plant and Equipment [Line Items]      
Property and equipment, net 36,900 38,400  
Accumulated depreciation 11,700 10,200  
Computer Equipment And Purchased Software      
Property, Plant and Equipment [Line Items]      
Property and equipment, gross 57,515 60,692  
Computer Equipment And Purchased Software | Assets Held under Capital Leases      
Property, Plant and Equipment [Line Items]      
Accumulated depreciation 600 2,200  
Property and equipment, gross $ 600 $ 2,200