Business Combinations - Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill		$ 776,368	$ 778,951
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
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
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract]
Measurement Period Adjustments, Accounts Receivable	0
Measurement Period Adjustments, Prepaid and other current assets	0
Measurement Period Adjustments, Deferred income tax assets	44
Measurement Period Adjustments, Total assets acquired	44
Measurement Period Adjustments, Accounts payable and accrued	(1,339)
Measurement Period Adjustments, Other non-current liabilities	(1,166)
Measurement Period Adjustments, Total liabilities assumed	(2,505)
Measurement Period Adjustments, Total identifiable net assets	2,549
Measurement Period Adjustments, Goodwill	(2,549)
Measurement Period Adjustments, Total consideration paid, net of cash acquired	$ 0