Accumulated Other Comprehensive Income (Loss) (Components of Accumulated Other Comprehensive Loss) (Details) - Accumulated Defined Benefit Plans Adjustment Including Portion Attributable to Noncontrolling Interest [Member] - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, net of tax	$ (7,531)	$ (7,567)
Pension Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, net of tax	(9,618)	(9,707)
Accumulated other comprehensive loss, tax	6,096	6,120
Postretirement Benefit Plan [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, net of tax	2,087	2,140
Accumulated other comprehensive loss, tax	$ 519	$ 533

Accumulated Other Comprehensive Income (Loss) (Components of Accumulated Other Comprehensive Loss) (Details) - Accumulated Defined Benefit Plans Adjustment Including Portion Attributable to Noncontrolling Interest [Member] - USD ($)
$ in Thousands

Mar. 31, 2018

Dec. 31, 2017

Accumulated Other Comprehensive Income (Loss) [Line Items]

Accumulated other comprehensive loss, net of tax

$ (7,531)

$ (7,567)

Pension Benefits [Member]

Accumulated Other Comprehensive Income (Loss) [Line Items]

Accumulated other comprehensive loss, net of tax

(9,618)

(9,707)

Accumulated other comprehensive loss, tax