Employee Benefits (Retirement plan assets) (Details 4)	12 Months Ended
Dec. 31, 2017 D
Asset allocation guidelines
Effect of market fluctuations in cash flow on target allocation limits (as a percent)	5.00%
Period during which asset allocation is expected to conform to range limits (in days)	90
Minimum
Asset allocation guidelines
Fixed income securities, percentage of variable component in strategic targets (as a percent)	10.00%
Maximum
Asset allocation guidelines
Fixed income securities, percentage of variable component in strategic targets (as a percent)	20.00%
S&P 500 Index
Defined benefit pension plan disclosure
Performance benchmark (as a percent)	36.00%
MSCI EAFE Index
Defined benefit pension plan disclosure
Performance benchmark (as a percent)	24.00%
Barclays Capital Aggregate Bond Index
Defined benefit pension plan disclosure
Performance benchmark (as a percent)	40.00%
Equity Securities
Defined benefit pension plan disclosure
Target plan asset allocations	60.00%
Equity Securities, Domestic \| Minimum
Defined benefit pension plan disclosure
Target plan asset allocations	50.00%
Equity Securities, Domestic \| Maximum
Defined benefit pension plan disclosure
Target plan asset allocations	100.00%
Equity Securities, International \| Minimum
Defined benefit pension plan disclosure
Target plan asset allocations	0.00%
Equity Securities, International \| Maximum
Defined benefit pension plan disclosure
Target plan asset allocations	50.00%
Fixed Income Securities
Defined benefit pension plan disclosure
Target plan asset allocations	40.00%
Cash \| Minimum
Defined benefit pension plan disclosure
Target plan asset allocations	0.00%
Cash \| Maximum
Defined benefit pension plan disclosure
Target plan asset allocations	20.00%

Performance percentage benchmarked to indexes.

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Represents the period during which the asset allocation affected by market fluctuations is expected to conform to range limits.

+ References

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Represents the breach limits allowed for each asset class due to market fluctuations or cash flows.

+ References

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- Definition

Target allocation percentage of variable component in strategic targets, of investments in debt securities to total plan assets presented on a weighted-average basis as of the measurement date of the latest statement of financial position.

+ References

No definition available.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Target allocation of investments of this type to total plan assets presented on a weighted average basis.

+ References

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)(i)
-URI http://asc.fasb.org/extlink&oid=65897934&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)(i)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

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