Employee Benefit Plans - Net Periodic Pension and Postretirement Benefit Costs (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service costs	$ 760	$ 680	$ 600
Interest costs	1,760	1,610	1,620
Expected return on plan assets	(2,070)	(1,810)	(1,720)
Amortization of prior service cost	0	0	0
Settlements and curtailments gain	0	0	190
Amortization of net (gain)/loss	1,120	1,280	1,070
Net periodic benefit cost (income)	1,570	1,760	1,760
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service costs	0	0	0
Interest costs	30	40	50
Expected return on plan assets	0	0	0
Amortization of prior service cost	0	0	(200)
Settlements and curtailments gain	0	0	(1,490)
Amortization of net (gain)/loss	(90)	(80)	(80)
Net periodic benefit cost (income)	$ (60)	$ (40)	$ (1,720)