Pension and Other Postretirement Benefits (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 1,766	$ 1,498	$ 3,532	$ 2,996
Interest cost	3,070	3,129	6,140	6,258
Expected return on plan assets	(4,235)	(4,031)	(8,470)	(8,062)
Amortization of prior service cost	66	64	132	128
Amortization of net actuarial loss	2,434	2,313	4,868	4,626
Net periodic benefit cost	3,101	2,973	6,202	5,946
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	2,486	2,018	4,972	4,036
Interest cost	7,198	7,253	14,396	14,506
Amortization of prior service cost	307	261	614	522
Amortization of net actuarial loss	4,734	3,681	9,468	7,362
Net periodic benefit cost	$ 14,725	$ 13,213	$ 29,450	$ 26,426