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Stock awards (Details 2) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Share-based Payment Arrangement, Noncash Expense [Abstract] |  |  |
Nonemployee director stock compensation shares issued | 45,273 | 41,644 | 38,605
Fair value of nonemployee director stock compensation shares issued | $ 1.1 | $ 1.2 | $ 1.0

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