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Stock-Based Compensation - Employee Stock Purchase Plans (Details) | 12 Months Ended |
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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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- Definition Amount of expense for award under share-based payment arrangement. Excludes amount capitalized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5047-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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