v3.10.0.1
Significant Accounting Policies (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Short-term Investments      
Short-term investments $ 25,000   $ 0
Non-Cash Long-Lived Asset Additions      
Capital asset additions during the period 70,700 $ 49,600  
Non-cash capital expenditure 6,100 $ 12,300  
Accounts payable and accrued expenses      
Marketing and Sales Costs      
Accrued marketing costs $ 16,100   $ 11,700
Minimum      
Amortizable Intangible Assets      
Useful life of capitalized content development costs 4 years    
Maximum      
Amortizable Intangible Assets      
Useful life of capitalized content development costs 5 years    
Graduate Program Segment | Minimum      
Performance Obligations      
Period of contract with university clients 10 years    
Graduate Program Segment | Maximum      
Performance Obligations      
Period of contract with university clients 15 years    
Short Course Segment | Minimum      
Performance Obligations      
Period of contract with university clients 42 days    
Short Course Segment | Maximum      
Performance Obligations      
Period of contract with university clients 112 days