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Employee Benefits - Retirement plan contribution (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Employee Benefits |  |  |
Retirement plan contributions | $ 5,126 | $ 4,435 | $ 4,067
Total ESOP shares outstanding | 1,170,611 | 1,307,439 | 1,334,372
Salary Reduction Plan |  |  |
Employee Benefits |  |  |
Retirement plan contributions | $ 2,960 | $ 2,599 | $ 2,402
ESOP |  |  |
Employee Benefits |  |  |
Retirement plan contributions | $ 2,166 | $ 1,836 | $ 1,665

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- Definition Amount of cost for defined contribution plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
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- Definition Sum of the allocated, committed-to-be-released and suspense shares of the entity held by the plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 40 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=109244661&loc=d3e17540-113929 + Details
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