Benefit Plans - Components of Net Periodic Benefit Cost (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Pension Benefits [Member]
Components of net periodic benefit cost
Service cost	$ 10,854		$ 9,994		$ 10,902
Interest cost	31,491		30,723		28,947
Expected return on plan assets	(40,735)		(38,412)		(36,973)
Amortization of prior service cost	259	[1]	197	[1]	271	[1]
Recognized net actuarial loss	9,497		5,688		3,345
Net periodic pension benefit cost	11,366		8,190		6,492
Other Benefits [Member]
Components of net periodic benefit cost
Service cost	0		0		1
Interest cost	930		993		1,127
Expected return on plan assets	0		0		0
Amortization of prior service cost	(517)	[1]	(517)	[1]	(517)	[1]
Recognized net actuarial loss	389		284		442
Net periodic pension benefit cost	$ 802		$ 760		$ 1,053

[1] The annual amortization of pension benefits 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.