Employee Retirement Plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Pension cost
Components of net periodic cost:
Service cost	$ 67	$ 240	$ 134	$ 480
Interest cost	370	385	740	770
Expected return on plan assets	(318)	(339)	(636)	(678)
Amortization of net (gain) loss	192	113	384	226
Net periodic cost	311	399	622	798
Cash contributions			838	1,222
Pension cost | Annual estimate | Minimum
Components of net periodic cost:
Estimated total required contributions	2,512		2,512
Other postretirement benefit cost
Components of net periodic cost:
Service cost	70	66	140	131
Interest cost	90	102	180	204
Amortization of net (gain) loss	(4)	(15)	(8)	(31)
Net periodic cost	$ 156	$ 153	$ 312	$ 304