Employee Retirement Plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Pension cost
Components of net periodic cost:
Service cost	$ 62	$ 67	$ 185	$ 201
Interest cost	362	370	1,087	1,110
Expected return on plan assets	(352)	(318)	(1,057)	(954)
Amortization of net (gain) loss	302	192	905	576
Net periodic cost	374	311	1,120	933
Cash contributions			584	2,014
Pension cost | Minimum | Annual estimate
Components of net periodic cost:
Estimated total required contributions	828		828
Other postretirement benefit cost
Components of net periodic cost:
Service cost	63	70	188	210
Interest cost	61	90	184	270
Amortization of net (gain) loss	(43)	(4)	(129)	(12)
Net periodic cost	$ 81	$ 156	$ 243	$ 468