Retirement Plans (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Retirement Plans (Textuals) [Abstract]
Retirement eligible age for participation	55		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	2,612,000	2,613,000	5,225,000	5,225,000	9,898,000
Interest cost	1,612,000	1,589,000	3,225,000	3,180,000	6,414,000
Expected return on plan assets	(3,557,000)	(3,220,000)	(7,115,000)	(6,440,000)	(12,896,000)
Amortization of prior service cost	18,000	17,000	35,000	35,000	72,000
Amortization of actuarial loss	640,000	452,000	1,280,000	903,000	2,024,000
Total net periodic benefit cost	1,325,000	1,451,000	2,650,000	2,903,000	5,512,000
Other Postretirement Benefit Plans, Defined Benefit [Member]
Net period benefit cost for the pension and postretirement plans
Interest cost	48,000	50,000	95,000	100,000	198,000
Amortization of prior service cost	42,000	99,000	85,000	198,000	395,000
Amortization of actuarial loss	0	0	0	0	0
Total net periodic benefit cost	$ 90,000	$ 149,000	$ 180,000	$ 298,000	$ 593,000