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Employee Benefit Plans (Narrative) (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2007
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Deferred Compensation Cash-based Arrangements, Liability, Current and Noncurrent | $ 33,400 | $ 28,400 |  |
Defined Contribution Plan, Cost | $ 25,300 | 23,500 | $ 22,600 |
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] |  |  |  |
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment | $ 0 | 0 | $ (2,076) |
Defined Benefit Plan, Accumulated Benefit Obligation | $ 157,613 | $ 143,676 |  |
Assumed discount rate (percent) | 4.30% | 4.68% | 4.27% |
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 | $ 659,023 | $ 587,327 |  |
Employer contributions | $ 50,000 | $ 50,000 |  |
Assumed discount rate (percent) | 4.30% | 4.68% | 4.27% |
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% |
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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=82843861&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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=82843861&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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=82843861&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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(ii) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2709-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(iv)(01) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of (increase) decrease in benefit obligation of defined benefit plan from event reducing expected years of future service of present employees or eliminating accrual of benefits for some or all future services of present employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(10) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2446-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of cost for defined contribution plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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 Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
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