Retirement Plans, Accumulated Benefit Obligation in Excess of Plan Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Pension Plan [Member]
Accumulated benefit obligation in excess of plan assets [Abstract]
Projected benefit obligation at end of year	$ 354,994	$ 337,176
Accumulated benefit obligation at end of year	324,904	272,591
Fair value of plan assets at end of year	294,995	257,517
SERP [Member]
Accumulated benefit obligation in excess of plan assets [Abstract]
Projected benefit obligation at end of year	1,412	1,457
Accumulated benefit obligation at end of year	1,412	1,457
Fair value of plan assets at end of year	$ 0	$ 0

No definition available.

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- Definition

Amount of accumulated benefit obligation for defined benefit pension plan with accumulated benefit obligation in excess of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2417-114920

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- Definition

Fair value of plan assets for defined benefit pension plan with accumulated benefit obligation in excess of plan assets.

+ References

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- Definition

Amount of projected benefit obligation for defined benefit pension plan with accumulated benefit obligation in excess of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=77993564&loc=d3e4179-114921

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