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Employee Benefit Plans (Details) | 12 Months Ended
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Dec. 31, 2017 USD ($) installment $ / shares shares | Dec. 31, 2016 USD ($) $ / shares shares | Dec. 31, 2015 USD ($) $ / shares shares
Defined Benefit Plan Disclosure |  |  |
Employer contribution for defined contribution plan | $ 2,800,000 | $ 2,000,000 | $ 1,000,000
Employee Stock Ownership Plan, common stock, shares outstanding (shares) | shares | 145,598.175 |  |
Employee Stock Ownership Plan, common stock, fair value | $ 14,600,000 |  |
Outside Directors |  |  |
Defined Benefit Plan Disclosure |  |  |
Number of deferred common stock (shares) | shares | 12,293 | 10,166 | 8,466
Weighted-average grant date fair values of deferred common stock (dollars per share) | $ / shares | $ 101.34 | $ 97.99 | $ 78.62
Nonqualified Deferred Compensation Plan |  |  |
Defined Benefit Plan Disclosure |  |  |
Deferred compensation expense | $ 249,000 | $ 96,000 | $ 106,000
Number of annual installments of deferred compensation (installment) | installment | 2 |  |
Number of days after departure from board of directors for payment of deferred compensation (days) | 90 days |  |

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- Definition Fair value of unearned shares held by the employee stock ownership plan (ESOP). + 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 (e) -URI http://asc.fasb.org/extlink&oid=109244661&loc=d3e17540-113929 + 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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