Investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Available For Sale Debt Securities [Line Items]
Debt securities with amortized cost	$ 807.3
Debt securities with fair value	807.7	915.7
Short term investments	727.0
January 2014 Sales [Member]
Available For Sale Debt Securities [Line Items]
Debt securities with amortized cost	567.5
Debt securities with fair value	567.8
Short term investments	$ 161.2

Amount of available-for-sale debt securities at fair value maturing in the next fiscal year following the latest fiscal year.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27198-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 320

-Section 50

-Paragraph 3

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62586-112803

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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