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Retirement And Other Postretirement Benefit Plans (Narrative) (Details) (USD $) In Thousands, unless otherwise specified | 1 Months Ended | 3 Months Ended | 12 Months Ended |
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Jan. 31, 2011 | Mar. 31, 2012 | Mar. 31, 2011 | Mar. 31, 2008 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2008 | Dec. 31, 2007 | Dec. 31, 2006 Y
Retirement And Other Postretirement Benefit Plans [Abstract] |  |  |  |  |  |  |  |  |
Net unrecognized service credit |  |  |  |  |  |  |  |  | $ 14,079
Number of years for which net unrecognized service credit is being amortized |  |  |  |  |  |  |  |  | 13.5
Unrecognized prior service credit as a reduction to expense |  |  |  |  |  | 10,543 |  |  |
Number of supplemental non-qualified retirement plans sponsored by the Corporation |  | 2 |  |  |  |  |  |  |
Number of consecutive calendar years used to calculate BRP benefits |  |  |  |  |  |  | 5 |  |
Number of years used to calculate BRP benefits |  |  |  |  |  |  | 10 |  |
Percentage of employer match |  |  |  |  | 100.00% | 50.00% |  |  |
Employee contribution percentage |  |  |  |  | 4.00% | 6.00% |  |  |
Annuity equivalent of contribution to defined contribution plan deducted from monthly benefit of basic retirement plan |  | 3.00% |  |  |  |  |  |  |
Increase in annuity equivalent of contribution to defined contribution plan deducted from monthly benefit of basic retirement plan for full-time employees |  |  |  | 4.00% |  |  |  |  |
Additional automatic contribution increase, performance goals, percentage |  |  |  |  |  |  |  | 2.00% |
Contribution expense of the Corporation |  | $ 2,004 | $ 2,142 |  |  |  |  |  |
Automatic contribution increase available under contribution method, percentage | 3.00% |  |  |  |  |  |  | 2.00% |

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- Definition Annuity equivalent of contribution to defined contribution plan deducted from monthly benefit of basic retirement plan. + References No definition available. + Details
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- Definition Defined benefit plan unrecognized service cost amortization period. + References No definition available. + Details
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- Definition Increased annuity equivalent of contribution to defined contribution plan deducted from monthly benefit of basic retirement plan. + References No definition available. + Details
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- Definition Number of supplemental non qualified retirement plans. + References No definition available. + Details
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- Definition Post Retirement Benefits, Benefit Calculation, Number Of Consecutive Years + References No definition available. + Details
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- Details
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- Definition Amount related to the pretax cost of benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation, which has not yet been recognized as components of net periodic benefit cost. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 87 -Paragraph 264 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph i -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Prior Service Cost -URI http://asc.fasb.org/extlink&oid=6521920 -Topic 715 -SubTopic 60 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 158 -Paragraph 7 -Subparagraph c -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 106 -Paragraph 518 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Prior Service Cost -URI http://asc.fasb.org/extlink&oid=6521884 -Topic 715 -SubTopic 30 + Details
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- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 11 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: 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=6414718&loc=d3e28014-114942 + Details
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