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Business Combinations - Narrative (Details) - Community Education Centers - USD ($)
$ in Millions
3 Months Ended
Apr. 05, 2017
Mar. 31, 2018
Business Acquisition [Line Items]    
Percentage of voting interests acquired 100.00%  
Acquisition of CEC, net of cash acquired $ 353.6  
Measurement period adjustment   $ 1.0
Measurement period adjustment, valuation of an unfavorable contract liability   $ 1.4