Retirement Plans (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Retirement Plans (Textuals) [Abstract]
Retirement eligible age for participation	55
Contribution made by the Corporation to pension plan	$ 10,000,000
Pension Plans, Defined Benefit [Member]
Net period benefit cost for the pension and postretirement plans
Service cost	2,613,000	2,613,000	9,898,000
Interest cost	1,613,000	1,589,000	6,414,000
Expected return on plan assets	(3,558,000)	(3,220,000)	(12,896,000)
Amortization of prior service cost	17,000	18,000	72,000
Amortization of actuarial loss	640,000	451,000	2,024,000
Total net periodic benefit cost	1,325,000	1,451,000	5,512,000
Other Postretirement Benefit Plans, Defined Benefit [Member]
Net period benefit cost for the pension and postretirement plans
Interest cost	47,000	50,000	198,000
Amortization of prior service cost	43,000	99,000	395,000
Total net periodic benefit cost	$ 90,000	$ 149,000	$ 593,000