Retirement Plans and Other Postretirement Benefits - Components of Retirement Benefit Cost (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Retirement Plans [Member]
Components of net periodic benefit cost
Service cost	$ 622	$ 556	$ 362
Interest cost	1,726	1,725	1,708
Expected return on plan assets	(2,254)	(1,895)	(1,670)
Amortization of net loss	1,147	728	686
Accretion of prior service cost	(235)	(235)	(236)
Net periodic benefit cost	1,006	879	850
Other Post Retirement Benefits [Member]
Components of net periodic benefit cost
Service cost	82	66	76
Interest cost	116	117	115
Expected return on plan assets
Amortization of net loss	23	15	7
Accretion of prior service cost	(20)	(20)	(20)
Net periodic benefit cost	$ 201	$ 178	$ 178