Summary of Significant Accounting Policies - Disaggregation of revenue (Details) $ in Thousands	3 Months Ended								6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2018 item	Jun. 30, 2019 USD ($) item	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
Summary of Significant Accounting Policies
Number of lines of business | item									3	3
Total Revenues	$ 256,314	$ 253,252	$ 254,872	$ 251,314	$ 238,874	$ 232,864	$ 217,211	$ 228,785		$ 1,015,752	$ 917,734	$ 888,519
Managed Public School Programs
Summary of Significant Accounting Policies
Total Revenues										890,275
Institutional
Summary of Significant Accounting Policies
Total Revenues										89,953
Non-managed Public School Programs
Summary of Significant Accounting Policies
Total Revenues										50,623
Institutional Software & Services
Summary of Significant Accounting Policies
Total Revenues										39,330
Private Pay Schools and Other
Summary of Significant Accounting Policies
Total Revenues										$ 35,524