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Stock-Based Compensation - Employee Stock Purchase Plans (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) Stock_PurchasePlan $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2020 GBP (£) shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of employee stock purchase plans | Stock_PurchasePlan	3
United States Stock Repurchase Program | Class A Non-Voting
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized (in shares)	1,200,000			1,200,000
Maximum annual earnings withheld to purchase shares | $	$ 25,000
Purchase price of stock, percent of market price (percent)	85.00%
Shares issued (in shares)	114,408	131,100	143,769
Purchase price of shares during period (in usd per share) | $ / shares	$ 6.71	$ 7.38	$ 7.32
Projected exercises in period (in shares)	151,000			151,000
Share-based compensation expense | $	$ 343,000	$ 277,000	$ 321,000
U.K. Stock Repurchase Plan | Class A Non-Voting
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized (in shares)	1,200,000			1,200,000
Purchase price of stock, percent of market price (percent)	85.00%
Shares issued (in shares)	2,061	289,901	63,033
Share-based compensation expense | $	$ 163,000,000	$ 148,000,000	$ 140,000,000
Maximum monthly earnings withheld to purchase shares (in gbp) | £				£ 250
Estimated shares eligible for purchase (in shares)	108,000			108,000
U.K. Stock Repurchase Plan | Class A Non-Voting | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Purchase price of shares during period (in usd per share) | $ / shares	$ 6.32
U.K. Stock Repurchase Plan | Class A Non-Voting | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Purchase price of shares during period (in usd per share) | $ / shares	$ 8.95
International stock based compensation plan | Class A Non-Voting
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized (in shares)	1,000,000			1,000,000
Shares issued (in shares)	4,051	4,264	8,740
Purchase price of stock, percent of market price (percent)	85.00%
International stock based compensation plan | Class A Non-Voting | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum annual earnings withheld to purchase shares | $	$ 21,250
International stock based compensation plan | Class A Non-Voting | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum number of shares per employee	5,000

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