Fair Value Measurements (Details 1) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended
		Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Assets
Money Market Investments		$ 178,100	$ 178,100	$ 45,200
Marketable equity securities		491,172	491,172	585,080
Liabilities
Non-cash accretion on contingent consideration		800	800
Contingent Consideration settlement		5,700	5,700
Fair Value, Recurring [Member]
Assets
Money Market Investments	[1]	178,054	178,054	45,150
Marketable equity securities	[2]	491,172	491,172	585,080
Other current investments	[3]	15,074	15,074	14,887
Interest Rate Swap	[4]			131
Total Financial Assets		684,300	684,300	645,248
Liabilities
Deferred compensation plan liabilities	[5]	28,484	28,484	34,674
Contingent consideration liabilities	[6]	45,687	45,687
Interest rate swap	[7]	2,624	2,624	1,119
Foreign exchange swap	[8]			273
Mandatorily redeemable noncontrolling interest	[9]	768	768	829
Total Financial Liabilities		77,563	77,563	36,895
Fair Value, Recurring [Member] | Level 1
Assets
Money Market Investments	[1]	0	0	0
Marketable equity securities	[2]	491,172	491,172	585,080
Other current investments	[3]	12,008	12,008	8,843
Interest Rate Swap	[4]			0
Total Financial Assets		503,180	503,180	593,923
Liabilities
Deferred compensation plan liabilities	[5]	0	0	0
Contingent consideration liabilities	[6]	0	0
Interest rate swap	[7]	0	0	0
Foreign exchange swap	[8]			0
Mandatorily redeemable noncontrolling interest	[9]	0	0	0
Total Financial Liabilities		0	0	0
Fair Value, Recurring [Member] | Level 2
Assets
Money Market Investments	[1]	178,054	178,054	45,150
Marketable equity securities	[2]	0	0	0
Other current investments	[3]	3,066	3,066	6,044
Interest Rate Swap	[4]			131
Total Financial Assets		181,120	181,120	51,325
Liabilities
Deferred compensation plan liabilities	[5]	28,484	28,484	34,674
Contingent consideration liabilities	[6]	0	0
Interest rate swap	[7]	2,624	2,624	1,119
Foreign exchange swap	[8]			273
Mandatorily redeemable noncontrolling interest	[9]	0	0	0
Total Financial Liabilities		31,108	31,108	36,066
Fair Value, Recurring [Member] | Level 3
Assets
Money Market Investments	[1]	0	0	0
Marketable equity securities	[2]	0	0	0
Other current investments	[3]	0	0	0
Interest Rate Swap	[4]			0
Total Financial Assets		0	0	0
Liabilities
Deferred compensation plan liabilities	[5]	0	0	0
Contingent consideration liabilities	[6]	45,687	45,687
Interest rate swap	[7]	0	0	0
Foreign exchange swap	[8]			0
Mandatorily redeemable noncontrolling interest	[9]	768	768	829
Total Financial Liabilities		$ 46,455	$ 46,455	$ 829

[1] The Company’s money market investments are included in cash and cash equivalents and the value considers the liquidity of the counterparty.

[2] The Company’s investments in marketable equity securities are held in common shares of U.S. and Canadian corporations that are actively traded on U.S. and Canadian stock exchanges. Price quotes for these shares are readily available.

[3] Includes U.S. Government Securities, corporate bonds, mutual funds and time deposits. These investments are valued using a market approach based on the quoted market prices of the security or inputs that include quoted market prices for similar instruments and are classified as either Level 1 or Level 2 in the fair value hierarchy.

[4] Included in Other current assets. The Company utilized a market approach model using the notional amount of the interest rate swap multiplied by the observable inputs of time to maturity and market interest rates.

[5] Includes Graham Holdings Company’s Deferred Compensation Plan and supplemental savings plan benefits under the Graham Holdings Company’s Supplemental Executive Retirement Plan, which are included in accrued compensation and related benefits. These plans measure the market value of a participant’s balance in a notional investment account that is comprised primarily of mutual funds, which are based on observable market prices. However, since the deferred compensation obligations are not exchanged in an active market, they are classified as Level 2 in the fair value hierarchy. Realized and unrealized gains (losses) on deferred compensation are included in operating income.

[6] Included in Accounts payable and accrued liabilities and Other Liabilities. The Company determined the fair value of the contingent consideration using a Monte Carlo simulation as of the acquisition date, which included using estimated financial projections for the acquired business. The Company has subsequently recorded a $0.8 million non-cash accretion of the contingent consideration and settled $5.7 million of the liability in the three and nine months ended September 30, 2020.

[7] Included in Other Liabilities. The Company utilized a market approach model using the notional amount of the interest rate swap multiplied by the observable inputs of time to maturity and market interest rates.

[8] Included in Accounts payable and accrued liabilities, and valued based on a valuation model that calculates the differential between the contract price and the market-based forward rate.

[9] The fair value of the mandatorily redeemable noncontrolling interest is based on the fair value of the underlying subsidiaries owned by GHC One (see Note 2), after taking into account any debt and other noncontrolling interests of its subsidiary investments. The fair value of the owned subsidiaries is determined by reference to either a discounted cash flow or EBITDA multiple, which approximates fair value.