Fair Value Measurements (Fair Value of Financial Assets and Liabilities) (Details 1) - USD ($) $ in Thousands		Mar. 31, 2021	Dec. 31, 2020
Assets
Money Market Investments		$ 175,200	$ 268,800
Marketable equity securities		700,352	573,102
Fair Value, Recurring [Member]
Assets
Money Market Investments	[1]	175,200	268,841
Marketable equity securities	[2]	700,352	573,102
Other current investments	[3]	15,228	14,480
Total Financial Assets		890,780	856,423
Liabilities
Deferred compensation plan liabilities	[4]	29,442	31,178
Contingent consideration liabilities	[5]	35,822	37,174
Interest rate swap	[6]	1,720	2,342
Foreign exchange swap	[7]		259
Mandatorily redeemable noncontrolling interest	[8]	10,272	9,240
Total Financial Liabilities		77,256	80,193
Level 1 | Fair Value, Recurring [Member]
Assets
Money Market Investments	[1]	0	0
Marketable equity securities	[2]	700,352	573,102
Other current investments	[3]	10,121	10,397
Total Financial Assets		710,473	583,499
Liabilities
Deferred compensation plan liabilities	[4]	0	0
Contingent consideration liabilities	[5]	0	0
Interest rate swap	[6]	0	0
Foreign exchange swap	[7]		0
Mandatorily redeemable noncontrolling interest	[8]	0	0
Total Financial Liabilities		0	0
Level 2 | Fair Value, Recurring [Member]
Assets
Money Market Investments	[1]	175,200	268,841
Marketable equity securities	[2]	0	0
Other current investments	[3]	5,107	4,083
Total Financial Assets		180,307	272,924
Liabilities
Deferred compensation plan liabilities	[4]	29,442	31,178
Contingent consideration liabilities	[5]	0	0
Interest rate swap	[6]	1,720	2,342
Foreign exchange swap	[7]		259
Mandatorily redeemable noncontrolling interest	[8]	0	0
Total Financial Liabilities		31,162	33,779
Level 3 | Fair Value, Recurring [Member]
Assets
Money Market Investments	[1]	0	0
Marketable equity securities	[2]	0	0
Other current investments	[3]	0	0
Total Financial Assets		0	0
Liabilities
Deferred compensation plan liabilities	[4]	0	0
Contingent consideration liabilities	[5]	35,822	37,174
Interest rate swap	[6]	0	0
Foreign exchange swap	[7]		0
Mandatorily redeemable noncontrolling interest	[8]	10,272	9,240
Total Financial Liabilities		$ 46,094	$ 46,414

[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] 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.

[5] 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.

[6] 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.

[7] 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.

[8] The fair value of the mandatorily redeemable noncontrolling interest is based on the fair value of the underlying subsidiaries owned by GHC One, 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.