Benefit Plans - Components of Net Periodic Benefit Cost (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			12 Months Ended
	Sep. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Pension Benefits [Member]
Components of net periodic benefit cost
Service cost				$ 11,616		$ 11,446		$ 10,854
Interest cost				27,597		31,831		31,491
Expected return on plan assets				(35,746)		(40,821)		(40,735)
Amortization of prior service cost				(313)	[1]	258	[1]	259	[1]
Recognized net actuarial loss				14,469		16,777		9,497
Curtailment loss	(809)	(1,159)		10,672		0		0
Settlement loss			(21,907)	20,156		21,907		0
Special termination benefits				38,733		0		0
Net periodic pension benefit cost				87,184		41,398		11,366
Other Benefits [Member]
Components of net periodic benefit cost
Service cost				0		0		0
Interest cost				628		814		930
Expected return on plan assets				0		0		0
Amortization of prior service cost				(488)	[1]	(517)	[1]	(517)	[1]
Recognized net actuarial loss				423		488		389
Curtailment loss				0		0		0
Settlement loss				0		0		0
Special termination benefits				0		0		0
Net periodic pension benefit cost				$ 563		$ 785		$ 802

[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.