Benefit Plans - Components of Net Periodic Benefit Cost (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Pension Benefits [Member]
Components of net periodic benefit cost
Service cost	$ 11,446		$ 10,854		$ 9,994
Interest cost	31,831		31,491		30,723
Expected return on plan assets	(40,821)		(40,735)		(38,412)
Amortization of prior service cost	258	[1]	259	[1]	197	[1]
Recognized net actuarial loss	16,777		9,497		5,688
Settlement loss	21,907		0		0
Net periodic pension benefit cost	41,398		11,366		8,190
Other Benefits [Member]
Components of net periodic benefit cost
Service cost	0		0		0
Interest cost	814		930		993
Expected return on plan assets	0		0		0
Amortization of prior service cost	(517)	[1]	(517)	[1]	(517)	[1]
Recognized net actuarial loss	488		389		284
Settlement loss	0		0		0
Net periodic pension benefit cost	$ 785		$ 802		$ 760

[1] The annual amortization of prior service cost is determined as the increase in projected benefit obligation due to the plan change divided by the average remaining service period of participating employees expected to receive benefits under the plan.