Retirement Benefits (Additional Information) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer	$ 0.5
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Expected contribution for next fiscal year		1.6
Expected contributions in current fiscal year	5.7
Other Postretirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Expected contribution for next fiscal year		0.5
Expected contributions in current fiscal year	$ 0.6

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=35743120&loc=d3e1928-114920

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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=35743120&loc=d3e2709-114920

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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=35743120&loc=d3e1928-114920

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