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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 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 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 + Details
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