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Marketable Securities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable, Recorded Investment [Line Items]
Marketable securities	$ 334us-gaap_MarketableSecurities	$ 57us-gaap_MarketableSecurities	$ 238us-gaap_MarketableSecurities
U.S. Treasury Bills [Member]
Financing Receivable, Recorded Investment [Line Items]
Marketable securities			150us-gaap_MarketableSecurities / us-gaap_InvestmentTypeAxis = us-gaap_USTreasuryBillSecuritiesMember
U.S.Treasury Bonds [Member]
Financing Receivable, Recorded Investment [Line Items]
Marketable securities	334us-gaap_MarketableSecurities / us-gaap_InvestmentTypeAxis = us-gaap_USTreasuryBondSecuritiesMember
Weighted average maturity period	5 years 3 months 18 days
Aggregate amortized cost basis of securities	333us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_USTreasuryBondSecuritiesMember
Corporate Bonds [Member]
Financing Receivable, Recorded Investment [Line Items]
Marketable securities		57us-gaap_MarketableSecurities / us-gaap_InvestmentTypeAxis = us-gaap_CorporateBondSecuritiesMember	88us-gaap_MarketableSecurities / us-gaap_InvestmentTypeAxis = us-gaap_CorporateBondSecuritiesMember
Aggregate amortized cost basis of securities		$ 57us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_CorporateBondSecuritiesMember
Securities maturity year		2014

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- Definition

Marketable Securities Maturity Year.

+ References

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- Definition

Weighted Average Maturity Period

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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

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- Definition

Total debt and equity financial instruments including: (1) securities held-to-maturity, (2) trading securities, and (3) securities available-for-sale.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.4)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

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

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