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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
	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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Employee Contribution Percentage

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Increased annuity equivalent of contribution to defined contribution plan deducted from monthly benefit of basic retirement plan.

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Number of supplemental non qualified retirement plans.

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Post Retirement Benefits, Benefit Calculation, Number Of Consecutive Years

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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.

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