Components of Net Periodic Benefit Cost (Details) (USD $)	1 Months Ended		3 Months Ended
	Nov. 30, 2012 acre	Apr. 30, 2011 acre	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
Net periodic benefit costs for pension benefits
Interest cost			$ 720,000	$ 798,000
Expected return on plan assets			(726,000)	(717,000)
Recognized actuarial loss			228,000	185,000
Net expense			222,000	266,000
Security to support the unfunded liabilities of the Bargaining Unit and Hourly Employees (Bargaining Plan						5,200,000
Area of property pledged as security (in acres)	7,000	1,400
Period for release of property pledged as collateral	5 years	5 years
Additional security to support unfunded liabilities					18,700,000
Expected minimum required contributions			2,100,000
Contributions funded by employer			$ 400,000