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Employee Benefit Plans (Present Value of Accrued Liability) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Executives Directors And Officer Of Acquired Entities [Member] |  |  |
Present Value Of Accrued Liability [Roll Forward] |  |  |
Present value of accrued liability as of January 1 | $ 38,597 | $ 39,878 |
Benefit expense and interest cost | 3,262 | 3,232 |
Benefits paid | 4,560 | 4,194 |
Benefits forfeited | 0 | (319) |
Present value of accrued liability as of December 31 | 37,299 | 38,597 | $ 39,878
BancShares Plan |  |  |
Present Value Of Accrued Liability [Roll Forward] |  |  |
Benefit expense and interest cost | 8,569 | 11,936 | 19,446
Interest cost | $ 28,940 | $ 28,892 | $ 26,975
Discount rate | 3.76% | 4.30% |

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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation 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 Amount of cost recognized for passage of time related to 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 (h)(2) -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 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of net periodic benefit cost (credit) for 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 (h) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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