Retirement Plans (Changes in Accumulated Other Comprehensive Income (Loss) Related to Retirement Plans) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Accumulated Other Comprehensive Income (Loss), Net of Tax
Reclassification of certain income tax effects	$ 0	[1]
Pension Plan
Accumulated Other Comprehensive Income (Loss), Net of Tax
Accumulated other comprehensive income (loss) at beginning of year	(26,123)
Prior service costs (credits)	0
Net actuarial (income) loss	1,528
Curtailment	7,742
Settlement	0
Reclassification of certain income tax effects	(3,630)
Comprehensive income (loss) adjustment	9,270
Accumulated other comprehensive income (loss) at end of year	(20,483)
Postretirement Plan
Accumulated Other Comprehensive Income (Loss), Net of Tax
Accumulated other comprehensive income (loss) at beginning of year	724
Prior service costs (credits)	0
Net actuarial (income) loss	(112)
Curtailment	0
Settlement	0
Reclassification of certain income tax effects	132
Comprehensive income (loss) adjustment	(112)
Accumulated other comprehensive income (loss) at end of year	744
SERP
Accumulated Other Comprehensive Income (Loss), Net of Tax
Accumulated other comprehensive income (loss) at beginning of year	(495)
Prior service costs (credits)	0
Net actuarial (income) loss	229
Curtailment	0
Settlement	209
Reclassification of certain income tax effects	(12)
Comprehensive income (loss) adjustment	438
Accumulated other comprehensive income (loss) at end of year	(69)
Defined Benefit Pension Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax
Accumulated other comprehensive income (loss) at beginning of year	(25,894)
Prior service costs (credits)	0
Net actuarial (income) loss	1,645
Curtailment	7,742
Settlement	209
Reclassification of certain income tax effects	(3,510)
Comprehensive income (loss) adjustment	9,596
Accumulated other comprehensive income (loss) at end of year	$ (19,808)

[1] The reclassification from accumulated other comprehensive income (loss) to retained earnings was due to the early adoption of ASU 2018-02, refer to Note 1 for further details on the adoption.