Retirement Benefits (Details 7)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Benefit Obligation [Member]
Defined Benefit Plan Disclosure (Table Amounts)
Initial health care cost trend rate assumed for the next year	6.50%	6.50%
Rate to which the health care cost trend rate is assumed to decline (the ultimate trend rate)	5.00%	5.00%
Year that the health care cost trend rate reaches the ultimate trend rate	2023	2020
Pension Benefits
Assumptions Used to Determine Benefit Obligation
Discount rate	3.68%	4.19%
Initial cash balance crediting rate assumed for the next year	2.75%	3.10%
Rate to which the cash balance crediting rate is assumed to increase (the ultimate rate)	3.00%	3.60%
Year that the cash balance crediting rate reaches the ultimate rate	2023	2022
Rate of compensation increase	3.00%	3.00%
Assumptions Used to Determine Benefit Cost
Discount rate	4.19%	4.53%
Initial cash balance crediting rate assumed for the next year	3.10%	3.00%
Rate to which the cash balance crediting rate is assumed to increase (the ultimate rate)	3.60%	3.75%
Year that the cash balance crediting rate reaches the ultimate rate	2022	2021
Expected long-term return on plan assets	8.00%	8.00%
Rate of compensation increase	3.00%	3.00%
Post-retirement Benefit Cost [Member]
Defined Benefit Plan Disclosure (Table Amounts)
Initial health care cost trend rate assumed for the next year	6.50%	7.00%
Rate to which the health care cost trend rate is assumed to decline (the ultimate trend rate)	5.00%	5.00%
Year that the health care cost trend rate reaches the ultimate trend rate	2020	2020
Medical and Life Benefits
Assumptions Used to Determine Benefit Obligation
Discount rate	3.66%	4.13%
Assumptions Used to Determine Benefit Cost
Discount rate	4.13%	4.47%
Expected long-term return on plan assets	7.70%	7.70%

Cash Balance Crediting Rate Assumed for next fiscal year used to calculate benefit cost

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Defined Benefit Plan, Cash Balance Crediting Rate, Year that the rate reaches the ultimate rate, used to calculate benefit cost.

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Defined Benefit Plan Ultimate Cash Balance Crediting Rate, used to calculate benefit cost

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Cash Balance Crediting Rate Assumed for Next Fiscal Year used to calculate the benefit obligation.

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Defined Benefit Plan, Cash Balance Crediting Rate, Year that rate reaches ultimate rate, used to calculate benefit obligation, in CCYY format.

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Defined Benefit Plan Ultimate Cash Balance Crediting Rate used to calculate the benefit obligation

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
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Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

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

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Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

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Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

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-Name Accounting Standards Codification
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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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Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status.

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-Publisher FASB
-Name Accounting Standards Codification
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Ultimate trend rate for health care cost for defined benefit postretirement plan.

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Year ultimate health care cost trend rate is expected to be reached, in CCYY format.

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