Retirement benefits Weighted Average Asset Allocation of Defined Benefit Retirement Plans and Weighted Average Assumptions(Details)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation (as a percent)	100.00%	100.00%
Asset category, Target
Target (as a percent)	100.00%
Benefit obligation
Discount rate (as a percent)	4.22%	5.09%	4.13%
Rate of compensation increase (as a percent)	3.50%	3.50%	3.50%
Net periodic benefit cost (years ended)
Discount rate (as a percent)	5.09%	4.13%	5.19%
Expected return on plan assets (as a percent)	7.75%	7.75%	7.75%
Rate of compensation increase (as a percent)	3.50%	3.50%	3.50%
Pension benefits \| Equity securities managers
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation (as a percent)	73.00%	73.00%
Asset category, Target
Target (as a percent)	70.00%
Target minimum range (as a percent)	65.00%
Target maximum range (as a percent)	75.00%
Pension benefits \| Fixed income securities managers
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation (as a percent)	27.00%	27.00%
Asset category, Target
Target (as a percent)	30.00%
Target minimum range (as a percent)	25.00%
Target maximum range (as a percent)	35.00%
Other benefits
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation (as a percent)	100.00%	100.00%
Asset category, Target
Target (as a percent)	100.00%
Benefit obligation
Discount rate (as a percent)	4.17%	5.03%	4.07%
Net periodic benefit cost (years ended)
Discount rate (as a percent)	5.03%	4.07%	4.90%
Expected return on plan assets (as a percent)	7.75%	7.75%	7.75%
Other benefits \| Equity securities managers
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation (as a percent)	72.00%	74.00%
Asset category, Target
Target (as a percent)	70.00%
Target minimum range (as a percent)	65.00%
Target maximum range (as a percent)	75.00%
Other benefits \| Fixed income securities managers
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation (as a percent)	28.00%	26.00%
Asset category, Target
Target (as a percent)	30.00%
Target minimum range (as a percent)	25.00%
Target maximum range (as a percent)	35.00%

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The interest rate used to adjust for the time value of money for the plan.

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- Definition

Expected rate of compensation increases (for pay-related plans).

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The interest rate used to adjust for the time value of money.

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An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 30

-Glossary Expected Long-Term Rate of Return on Plan Assets

-URI http://asc.fasb.org/extlink&oid=6512013

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (k)(3)

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Expected rate of compensation increases (for pay-related plans).

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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 1

-Subparagraph (d)(i)

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-Section 50

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

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

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- Definition

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.

+ 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)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920

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-Publisher FASB

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-Section 50

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