v3.8.0.1
Significant Accounting Policies (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Mar. 31, 2016
Amortizable Intangible Assets      
Useful life of capitalized content development costs 5 years    
Non-Cash Long-Lived Asset Additions      
Capital asset additions during the period $ 62.3 $ 30.8  
Minimum      
Amortizable Intangible Assets      
Estimated useful life of intangible assets (in years) 3 years    
Maximum      
Amortizable Intangible Assets      
Estimated useful life of intangible assets (in years) 10 years    
Accounts payable and accrued expenses      
Marketing and Sales Costs      
Accrued marketing costs $ 11.7 5.6  
ASU 2016-09      
Recent Accounting Pronouncements      
Amount of cumulative-effect reduction to retained earnings with an offset to additional paid-in-capital     $ (0.1)
Computer hardware | Minimum      
Property and Equipment, Net      
Useful lives (in years) 3 years    
Computer hardware | Maximum      
Property and Equipment, Net      
Useful lives (in years) 5 years    
Furniture and office equipment | Minimum      
Property and Equipment, Net      
Useful lives (in years) 5 years    
Furniture and office equipment | Maximum      
Property and Equipment, Net      
Useful lives (in years) 7 years    
Leasehold improvements | Minimum      
Property and Equipment, Net      
Useful lives (in years) 4 years    
Leasehold improvements | Maximum      
Property and Equipment, Net      
Useful lives (in years) 11 years    
Internally-developed software      
Amortizable Intangible Assets      
Estimated useful life of intangible assets (in years) 3 years    
Landlord funded leasehold improvements      
Non-Cash Long-Lived Asset Additions      
Non-cash capital expenditure $ 11.2 6.4  
Graduate Program Segment | Minimum      
Revenue by segment      
Term of revenue contracts 10 years    
Graduate Program Segment | Maximum      
Revenue by segment      
Term of revenue contracts 15 years    
Short Course Segment      
Revenue by segment      
Allowance for Doubtful Accounts Receivable $ 0.3 $ 0.0