Pension and Other Postretirement Benefits (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 Benefits
Components of net periodic benefit cost:
Service cost	$ 1,766	$ 1,498	$ 5,298	$ 4,494
Interest cost	3,070	3,129	9,210	9,387
Expected return on plan assets	(4,235)	(4,031)	(12,705)	(12,093)
Amortization of prior service cost	66	64	198	192
Amortization of net actuarial loss	2,434	2,313	7,302	6,939
Net periodic benefit cost	3,101	2,973	9,303	8,919
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	2,486	2,018	7,458	6,054
Interest cost	7,200	7,253	21,596	21,759
Amortization of prior service cost	307	261	921	783
Amortization of net actuarial loss	4,714	3,681	14,182	11,043
Net periodic benefit cost	$ 14,707	$ 13,213	$ 44,157	$ 39,639