Retirement Plans and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Retirement Plans [Member]
Components of net periodic benefit cost
Service cost	$ 156	$ 158
Interest cost	431	431
Expected return on plan assets	(555)	(491)
Amortization of net loss	286	291
Accretion of prior service cost	(59)	(59)
Net periodic benefit cost	259	330
Other Post Retirement Benefits [Member]
Components of net periodic benefit cost
Service cost	21	21
Interest cost	29	30
Amortization of net loss	6	6
Accretion of prior service cost	(5)	(5)
Net periodic benefit cost	$ 51	$ 52