Pension and Other Postretirement Employee Benefits (Narrative) (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Feb. 20, 2018
Defined Benefit Plan Disclosure [Line Items]
Amortization reclassified from AOCL	[1]	$ 23,415
Nonqualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Postretirement Benefits Payments		2,600	$ 2,600
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortization reclassified from AOCL	[1]	23,787
Pension Plans [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets				$ 38,700
Projected benefit obligation				62,000
Contributions		52,100	5,300
Pension Plans [Member] | Nonqualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation				62,000
Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortization reclassified from AOCL	[1]	(372)
Accumulated benefit obligation				$ 13,500
Supplemental Pension Plans [Member] | Nonqualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributions		1,300	$ 1,200
Voluntary Contributions [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributions		44,000
Accumulated Deficit [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortization reclassified from AOCL		$ 23,300

[1] See Note 2: Recent Accounting Pronouncements discussing the $23.3 million reclassification from AOCL to accumulated deficit.