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Stockholders' Equity and Stock Plans - Dividends (Details) - USD ($) $ / shares in Units, $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Equity [Abstract] |  |  |  |  |
Cash dividends to common stockholders | $ 12,506 | $ 10,958 | $ 8,860 | [1] |
Cash dividends per common share (usd per share) | $ 0.92 | $ 0.80 | $ 0.64 |  |
Retained earnings | $ 219,747 | $ 203,227 |  |  |
Stockholders' equity | $ 358,253 | $ 336,788 | $ 316,407 |  | $ 297,025
Period used to determine amount available for payment of dividends based on restriction (in years) | 3 years |  |  |  |
Amount of retained earnings available for payment of dividends based on restriction | $ 30,600 |  |  |  |
Cash held | $ 5,300 |  |  |  |
[1] | Share and per share data have been adjusted to reflect the two-for-one stock split effective November 27, 2018.

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- Definition Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Excludes cash and cash equivalents within disposal group and discontinued operation. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 210 -Section 45 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=118262064&loc=SL116631418-115840 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 210 -Section 45 -Paragraph 21 -URI http://asc.fasb.org/extlink&oid=118262064&loc=SL116631419-115840 + Details
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