Benefit Plans - Components of Net Periodic Benefit Cost (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Pension Benefits
Components of net periodic benefit cost
Service cost				$ 2,924		$ 11,616		$ 11,446
Interest cost				22,999		27,597		31,831
Expected return on plan assets				(25,798)		(35,746)		(40,821)
Amortization of prior service cost				(156)	[1]	(313)	[1]	258	[1]
Recognized net actuarial loss				3,025		14,469		16,777
Curtailment loss		(809)	(1,159)	0		10,672		0
Settlement loss	(21,907)			0		20,156		21,907
Special termination benefits				0		38,733		0
Net periodic pension benefit cost				2,994		87,184		41,398
Other Benefits
Components of net periodic benefit cost
Service cost				0		0		0
Interest cost				627		628		814
Expected return on plan assets				0		0		0
Amortization of prior service cost				(226)	[1]	(488)	[1]	(517)	[1]
Recognized net actuarial loss				202		423		488
Curtailment loss				0		0		0
Settlement loss				0		0		0
Special termination benefits				0		0		0
Net periodic pension benefit cost				$ 603		$ 563		$ 785

[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.