Retirement And Other Postretirement Benefit Plans (Narrative) (Detail) (USD $) In Thousands, unless otherwise specified	1 Months Ended	6 Months Ended		12 Months Ended
	Jan. 31, 2012	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2008 Year	Dec. 31, 2007	Dec. 31, 2006 Year
Net unrecognized service credit								$ 14,079
Number of years for which net unrecognized service credit is being amortized								13,500
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
Annuity equivalent of contribution to defined contribution plan deducted from monthly benefit of basic retirement plan		3.00%
Percentage of employer match				100.00%
Employee contribution percentage				4.00%
Additional automatic contribution increase, performance goals, percentage							2.00%
Contribution expense of the Corporation		$ 4,093	$ 4,069
Automatic contribution increase available under contribution method, percentage	3.00%