Retirement Benefits (Additional Information) (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer	$ 5.0
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Expected contribution for next fiscal year		$ 1.6
Expected contributions in current fiscal year	6.1
Other Postretirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Expected contribution for next fiscal year		$ 0.5
Expected contributions in current fiscal year	$ 0.7

The increase in the fair value of plan assets from contributions made by the employer.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (b)(3)

-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

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- Definition

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+ References

No definition available.

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- Definition

Amount of the employer's best estimate of contributions expected to be paid to the plan in the current fiscal period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e2709-114920

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- Definition

Amount of employer's best estimate of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest statement of financial position.

+ 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 (g)

-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

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