Retirement Plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Retirement Plans (Textuals) [Abstract]
Retirement eligible age for participation	55
Pension Plans, Defined Benefit [Member]
Net period benefit cost for the pension and postretirement plans
Service cost	$ 2,975	$ 2,613	$ 10,287
Interest cost	1,548	1,613	6,547
Expected return on plan assets	(4,305)	(3,558)	(14,713)
Amortization of prior service cost	17	17	72
Amortization of actuarial loss	1,073	640	2,708
Settlement charge	0	0	408
Total net periodic benefit cost	1,308	1,325	5,309
Other Postretirement Benefit Plans, Defined Benefit [Member]
Net period benefit cost for the pension and postretirement plans
Interest cost	40	47	182
Amortization of prior service cost	0	43	170
Total net periodic benefit cost	$ 40	$ 90	$ 352