Fair Value Measurements (Fair Value of Financial Assets and Liabilities) (Details 1) - USD ($) $ in Thousands		Dec. 31, 2020	Dec. 31, 2019
Assets
Money market investments		$ 268,800	$ 45,200
Marketable equity securities		573,102	585,080
Fair Value, Recurring [Member]
Assets
Money market investments	[1]	268,841	45,150
Marketable equity securities	[2]	573,102	585,080
Other current investments	[3]	14,480	14,887
Interest rate swap	[4]		131
Total Financial Assets		856,423	645,248
Liabilities
Deferred compensation plan liabilities	[5]	31,178	34,674
Contingent consideration liabilities	[6]	37,174	13,546
Interest rate swap	[7]	2,342	1,119
Foreign exchange swap	[8]	259	273
Mandatorily redeemable noncontrolling interest	[9]	9,240	829
Total Financial Liabilities		80,193	50,441
Level 1 [Member] | Fair Value, Recurring [Member]
Assets
Money market investments	[1]	0	0
Marketable equity securities	[2]	573,102	585,080
Other current investments	[3]	10,397	8,843
Interest rate swap	[4]		0
Total Financial Assets		583,499	593,923
Liabilities
Deferred compensation plan liabilities	[5]	0	0
Contingent consideration liabilities	[6]	0	0
Interest rate swap	[7]	0	0
Foreign exchange swap	[8]	0	0
Mandatorily redeemable noncontrolling interest	[9]	0	0
Total Financial Liabilities		0	0
Level 2 [Member] | Fair Value, Recurring [Member]
Assets
Money market investments	[1]	268,841	45,150
Marketable equity securities	[2]	0	0
Other current investments	[3]	4,083	6,044
Interest rate swap	[4]		131
Total Financial Assets		272,924	51,325
Liabilities
Deferred compensation plan liabilities	[5]	31,178	34,674
Contingent consideration liabilities	[6]	0	0
Interest rate swap	[7]	2,342	1,119
Foreign exchange swap	[8]	259	273
Mandatorily redeemable noncontrolling interest	[9]	0	0
Total Financial Liabilities		33,779	36,066
Level 3 [Member] | Fair Value, Recurring [Member]
Assets
Money market investments	[1]	0	0
Marketable equity securities	[2]	0	0
Other current investments	[3]	0	0
Interest rate swap	[4]		0
Total Financial Assets		0	0
Liabilities
Deferred compensation plan liabilities	[5]	0	0
Contingent consideration liabilities	[6]	37,174	13,546
Interest rate swap	[7]	0	0
Foreign exchange swap	[8]	0	0
Mandatorily redeemable noncontrolling interest	[9]	9,240	829
Total Financial Liabilities		$ 46,414	$ 14,375

[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 liabilities using a Monte Carlo simulation as of the acquisition dates, which included using estimated financial projections for the acquired businesses.

[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 3), 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.