Summary of Significant Accounting Policies (Narrative) (Details)		3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 22, 2018	Mar. 22, 2018	Dec. 31, 2018 business	Dec. 31, 2018 business	Jun. 30, 2018
Summary of Significant Accounting Policies [Line Items]
Maximum Term of Contract				1 year
Graham Healthcare Group [Member] | Graham Holdings Company [Member]
Summary of Significant Accounting Policies [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent					90.00%
Hoover Treated Wood Products [Member] | Graham Holdings Company [Member]
Summary of Significant Accounting Policies [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent			97.72%	97.72%
Transferred over Time [Member]
Summary of Significant Accounting Policies [Line Items]
Percentage of Revenue				80.00%
Kaplan International [Member]
Summary of Significant Accounting Policies [Line Items]
Number of Performance Obligations				1
Number of business lines			1	1
Higher Education [Member]
Summary of Significant Accounting Policies [Line Items]
Number of Performance Obligations		1	1
Number of Performance Obligations In Contract With Tuition Cap		2
Transition and Operations Support Agreement Initial Term	30 years
Television Broadcasting [Member] | Advertising Revenue [Member]
Summary of Significant Accounting Policies [Line Items]
Number of Performance Obligations				1
Revenue Recognition, Maximum Period between Recognition of Revenue and Receipt of Payment				60 days
Television Broadcasting [Member] | Retransmission Revenue [Member]
Summary of Significant Accounting Policies [Line Items]
Number of Performance Obligations				1
Revenue Recognition, Maximum Period between Recognition of Revenue and Receipt of Payment				60 days
Manufacturing [Member]
Summary of Significant Accounting Policies [Line Items]
Number of business lines			4	4
Revenue Recognition, Maximum Period between Recognition of Revenue and Receipt of Payment				90 days
Manufacturing [Member] | Transferred over Time [Member]
Summary of Significant Accounting Policies [Line Items]
Percentage of Revenue				27.00%
Health Care [Member]
Summary of Significant Accounting Policies [Line Items]
Number of Performance Obligations				1
Revenue Recognition, Maximum Period between Recognition of Revenue and Receipt of Payment				60 days
Health Care [Member] | Other Healthcare Services [Member]
Summary of Significant Accounting Policies [Line Items]
Number of Performance Obligations				1
SocialCode [Member]
Summary of Significant Accounting Policies [Line Items]
Number of Performance Obligations				1
Revenue Recognition, Maximum Period between Recognition of Revenue and Receipt of Payment				100 days
Minimum [Member] | Machinery and Equipment [Member]
Summary of Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life (in years)				3 years
Minimum [Member] | Building [Member]
Summary of Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life (in years)				20 years
Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Investment maturity length				3 months
Amortized Intangible Assets, Useful Life (in years)				10 years
Maximum [Member] | Machinery and Equipment [Member]
Summary of Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life (in years)				20 years
Maximum [Member] | Building [Member]
Summary of Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life (in years)				50 years