Employee Benefit Plans - Defined Benefit Plans - Weighted-Average Asset Allocations (Details) - The Bank - Cash Balance Plan	Dec. 31, 2017	Dec. 31, 2016
Weighted average asset allocations by asset category
Weighted average asset allocation by asset category	100.00%	100.00%
Mutual funds-fixed income
Weighted average asset allocations by asset category
Weighted average asset allocation by asset category	39.00%	39.00%
Mutual funds-equity
Weighted average asset allocations by asset category
Weighted average asset allocation by asset category	61.00%	61.00%

Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2446-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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Data Type:	num:percentItemType
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Period Type:	instant