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Employee Benefit Plans (Narrative) (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2007
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Deferred Compensation Cash-based Arrangements, Liability, Current and Noncurrent | $ 46,400 | $ 33,400 |  |
Defined Contribution Plan, Cost | $ 28,600 | $ 25,300 | $ 23,500 |
Percent of employer match | 100.00% |  |  | 4.50%
Percentage of employee compensation matched | 6.00% |  |  |
Additional contribution if employed at end of year | 3.00% |  |  |
Bancorporation Plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Actual % of Plan Assets | 100.00% | 100.00% |  |
Defined Benefit Plan, Accumulated Benefit Obligation | $ 152,344 | $ 157,613 |  |
Assumed discount rate (percent) | 3.76% | 4.30% | 4.68% |
Assumed rate of salary increases | 4.00% | 4.00% | 4.00% |
Expected long-term rate of return on plan assets (percent) | 7.50% | 7.50% | 7.50% |
BancShares Plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Actual % of Plan Assets | 100.00% | 100.00% |  |
Defined Benefit Plan, Accumulated Benefit Obligation | $ 626,746 | $ 659,023 |  |
Employer contributions | $ 50,000 | $ 50,000 |  |
Assumed discount rate (percent) | 3.76% | 4.30% | 4.68% |
Assumed rate of salary increases | 4.00% | 4.00% | 4.00% |
Expected long-term rate of return on plan assets (percent) | 7.50% | 7.50% | 7.50% |
Cash and equivalents | Bancorporation Plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Actual % of Plan Assets | 1.85% | 2.34% |  |
Cash and equivalents | BancShares Plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Actual % of Plan Assets | 2.00% | 9.00% |  |
401(k) Matching Range 2 |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Percent of employer match |  |  |  | 50.00%
Percentage of employee compensation matched |  |  |  | 3.00%
401(k) Matching Range 1 |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Percent of employer match |  |  |  | 100.00%
Percentage of employee compensation matched |  |  |  | 3.00%

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- Definition Defined Contribution Plan, Employer Matching Contribution, Additional Percent of Employees' Gross Pay If Employed at Year End + References No definition available. + Details
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- Definition Aggregate carrying value as of the balance sheet date of the liabilities for deferred compensation arrangements. Represents currently earned compensation under cash arrangements (such as a profit-sharing plan, rabbi trust, and employee contract--excluding equity-based arrangements) that is not actually paid until a later date. + References No definition available. + Details
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- Definition Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition Amount of contribution received by defined benefit plan from employer which increases plan assets. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=112275985&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e2709-114920 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition Percentage of actual investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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