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Employee Benefits - Retirement plan contribution (Details) - USD ($) $ in Thousands	12 Months Ended
	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

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+ References

No definition available.

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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

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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

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