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Other Employee Benefit Plans - Shares purchased (Details) - ESPP - $ / shares | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Shares purchased (in shares) | 7,274 | 4,918 | 4,647
Average price per share (in dollars per share) | $ 112.65 | $ 126.83 | $ 100.34

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