Business Combinations - Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Apr. 05, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Goodwill		$ 776,359	$ 778,951		$ 615,433
Community Education Centers
Business Acquisition [Line Items]
Accounts Receivable	$ 32,869		32,869
Prepaid and other current assets	4,397		4,397
Property and equipment	126,510		126,510
Intangible assets	76,000		76,000	$ 76,000
Favorable lease assets	3,110		3,110
Deferred income tax assets	4,160		4,116
Other non-current assets	4,327		4,327
Total assets acquired	251,373		251,329
Accounts payable and accrued expenses	50,312		51,651
Unfavorable lease liabilities	1,299		1,299
Other non-current liabilities	9,313		10,479
Total liabilities assumed	60,924		63,429
Total identifiable net assets	190,449		187,900
Goodwill	163,107		165,656
Total consideration paid, net of cash acquired	353,556		$ 353,556
Measurement Period Adjustments
Accounts Receivable	0
Prepaid and other current assets	0
Deferred income tax assets	44
Total assets acquired	44
Accounts payable and accrued expenses	(1,339)
Other non-current liabilities	(1,166)
Total liabilities assumed	(2,505)
Total liabilities assumed	2,549
Goodwill	(2,549)
Total consideration paid, net of cash acquired	$ 0