Pension and Other Postretirement Benefit Plans (Schedule of Pension Plans with an Accumulated Benefit Obligation in Excess of Plan Assets) (Details) - Pension Benefits - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 644,142	$ 733,990
Accumulated benefit obligation	644,069	733,949
Fair value of plan assets	$ 578,347	$ 679,245