Retirement Plans, Summary of Funded Status at End of Year (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan [Member]
Funded status at end of year [Abstract]
Fair value of plan assets	$ 294,995	$ 257,517	$ 243,588
Benefit obligations	(354,994)	(337,176)	(297,325)
Funded status and amount recognized at end of year	(59,999)	(79,659)
SERP [Member]
Funded status at end of year [Abstract]
Fair value of plan assets	0	0	0
Benefit obligations	(1,412)	(1,457)	(1,526)
Funded status and amount recognized at end of year	(1,412)	(1,457)
Other Postretirement Benefits [Member]
Funded status at end of year [Abstract]
Fair value of plan assets	0	0	0
Benefit obligations	(679)	(675)	$ (891)
Funded status and amount recognized at end of year	$ (679)	$ (675)

Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

+ Details

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- Definition

Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64852052&loc=d3e1703-114919

+ Details

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- References

No definition available.

+ Details

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- Details

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- Details

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- Details

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