Retirement And Other Postretirement Benefit Plans (Narrative) (Detail) (USD $) In Thousands, unless otherwise specified	1 Months Ended	9 Months Ended		12 Months Ended
	Jan. 31, 2012	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2008 Y	Dec. 31, 2007	Dec. 31, 2006 Y
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
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		$ 6,664	$ 6,222
Automatic contribution increase available under contribution method, percentage	3.00%