Business Combinations - Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	Jul. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Apr. 05, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Goodwill		$ 778,951	$ 778,951		$ 615,433	$ 615,438
Community Education Centers
Business Acquisition [Line Items]
Accounts Receivable		32,869	32,869	$ 29,936
Prepaid and other current assets		4,397	4,397	5,032
Property and equipment		126,510	126,510	126,510
Intangible assets		76,000	76,000	76,000
Favorable lease assets		3,110	3,110	3,110
Deferred income tax assets		4,116	4,116	2,223
Other non-current assets		4,327	4,327	4,327
Total assets acquired		251,329	251,329	247,138
Accounts payable and accrued expenses		51,651	51,651	53,800
Unfavorable lease liabilities		1,299	1,299	1,299
Other non-current liabilities		10,479	10,479	9,917
Total liabilities assumed		63,429	63,429	65,016
Total identifiable net assets		187,900	187,900	182,122
Goodwill		165,656	165,656	172,343
Total consideration paid, net of cash acquired		353,556	353,556	$ 354,465
Measurement Period Adjustments
Accounts Receivable			2,933
Prepaid and other current assets			(635)
Deferred income tax assets			1,893
Total assets acquired			4,191
Accounts payable and accrued expenses			(2,149)
Other non-current liabilities		$ 5,100	562
Total liabilities assumed			(1,587)
Total liabilities assumed			5,778
Goodwill			(6,687)
Total consideration paid, net of cash acquired	$ 2,600		$ (909)