Pension and Other Postretirement Benefit Plans (Schedule of Components of Net Periodic Benefit Costs) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,948	$ 1,533	$ 8,256
Interest cost	25,265	25,048	31,655
Expected return on plan assets	(42,383)	(46,674)	(51,528)
Amortization of prior service credits	(502)	(502)	(579)
Recognized net actuarial loss	8,896	9,163	20,870
Curtailment loss	0	0	5
Settlement loss (gain)	10,153	22,196	19,053
Net periodic benefit cost (credit)	4,377	10,764	27,732
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0
Interest cost	23,945	24,241	27,238
Expected return on plan assets	0	0	0
Amortization of prior service credits	(2,405)	0	(336,327)
Recognized net actuarial loss	23,112	19,168	102,875
Curtailment loss	0	0	0
Settlement loss (gain)	0	0	(8,932)
Net periodic benefit cost (credit)	$ 44,652	$ 43,409	$ (215,146)