Defined Benefit Postretirement Plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Transition obligation amortization period	20 years
Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	$ 604	$ 756
Interest cost	722	774
Expected return on plan assets	(1,825)	(1,675)
Net amortization	603	859
Total cost (benefit)	104	714
Other Benefits [Member]
Components of net periodic benefit cost:
Service cost	6	5
Interest cost	34	39
Expected return on plan assets	0	0
Net amortization	223	(2)
Total cost (benefit)	$ 263	$ 42