Fair Value Disclosures - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
Sep. 30, 2013	Dec. 31, 2012
Fair value, measurements, nonrecurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair value of the debt	$ 361.9
US Treasury notes securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities, debt maturities, period	December of 2013	March and June of 2013
Available-for-sale securities, amortized cost	85.0	224.9
Available-for-sale securities, gross unrealized gain (loss)	$ 0	$ 0

Available-for-sale Securities, Debt Maturities, Period

No definition available.

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- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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- Definition

Amount of unrealized gain (loss) on investments in debt and equity securities classified as available-for-sale securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563

+ Details

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- Definition

Fair value portion of borrowing which can be exchanged for a specified number of another security at the option of the issuer or the holder, for example, but not limited to, the entity's common stock.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Convertible Security

-URI http://asc.fasb.org/extlink&oid=6509036

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