Pension and Other Postretirement Benefits (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 1,766	$ 1,498
Interest cost	3,070	3,129
Expected return on plan assets	(4,235)	(4,031)
Amortization of prior service cost	66	64
Amortization of net actuarial loss	2,434	2,313
Net periodic benefit cost	3,101	2,973
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	2,486	2,018
Interest cost	7,198	7,253
Amortization of prior service cost	307	261
Amortization of net actuarial loss	4,734	3,681
Net periodic benefit cost	$ 14,725	$ 13,213