Postretirement Benefit Plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Retirement Plans [Member]
Net periodic benefit cost:
Service cost	$ 1,298	$ 1,123
Interest cost	2,149	2,190
Expected return on assets	(3,007)	(3,080)
Amortization of:
Net loss	1,382	945
Unrecognized prior service cost	90	87
Net periodic benefit cost	1,912	1,265
Other Benefits [Member]
Net periodic benefit cost:
Service cost	307	250
Interest cost	573	646
Amortization of:
Net loss	306	332
Unrecognized prior service cost	(391)	(391)
Net periodic benefit cost	$ 795	$ 837