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Business Combinations - Narrative (Details) - Community Education Centers - USD ($) $ in Millions |  | 3 Months Ended
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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

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