Organization, Basis of Presentation and Recent Accounting Pronouncements (Recent Accounting Pronouncements) (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ 6,179,948		$ 6,179,948			$ 5,799,173	$ 5,791,724
Revenues	674,766	$ 657,225	2,006,879	$ 1,916,029
Income from Operations	60,739	26,950	170,579	86,944
Costs and Expenses	614,027	$ 630,275	1,836,300	$ 1,829,085
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings						$ 7,449
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings							$ 5,791,724
Revenues	100		(1,200)
Costs and Expenses	$ (1,800)		(2,600)
Retained Earnings [Member] | Accounting Standards Update 2016-01 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption					$ 194,900
Higher Education [Member] | Kaplan University Transaction [Member] | Education [Member] | Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues			(4,500)
Income from Operations			$ (4,500)