Benefit Plans - Defined Benefit Plans - Weighted-average Assumptions Used to Determine Net Periodic Cost (Details)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Supplemental Retirement Plan
Weighted-average Assumption Used to Determine Net Periodic Benefit Cost:
Discount rate	3.85%	4.00%
Split-Dollar Life Insurance Benefit Plan
Weighted-average Assumption Used to Determine Net Periodic Benefit Cost:
Discount rate	3.85%	4.00%

Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan.

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

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