Retirement Plans Components of Net Periodic Benefit Cost (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Retirement Plans (Textuals) [Abstract]
Retirement eligible age for participation	55		55
Contribution made by the Corporation to pension plan			$ 10,000,000
Years of service for pension plan eligibility			5
Pension Plans, Defined Benefit [Member]
Net period benefit cost for the pension and postretirement plans
Service cost	2,975,000	2,612,000	5,950,000	5,225,000	10,287,000
Interest cost	1,548,000	1,612,000	3,095,000	3,225,000	6,547,000
Expected return on plan assets	(4,305,000)	(3,557,000)	(8,610,000)	(7,115,000)	(14,713,000)
Amortization of prior service cost	17,000	18,000	35,000	35,000	72,000
Amortization of actuarial loss	1,073,000	640,000	2,145,000	1,280,000	2,708,000
Settlement charge	0	0	0	0	408,000
Total net periodic benefit cost	1,308,000	1,325,000	2,615,000	2,650,000	5,309,000
Other Postretirement Benefit Plans, Defined Benefit [Member]
Net period benefit cost for the pension and postretirement plans
Interest cost	40,000	48,000	80,000	95,000	182,000
Amortization of prior service cost	0	42,000	0	85,000	170,000
Total net periodic benefit cost	$ 40,000	$ 90,000	$ 80,000	$ 180,000	$ 352,000