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Fair Value of Financial Instruments and Concentration of Credit Risk - Additional Information (Detail) (Credit Concentration Risk [Member], USD $) In Millions, unless otherwise specified | Dec. 31, 2014
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Certificate of Deposits [Member] |
Concentration Risk [Line Items] |
Marketable securities | $ 137.6 us-gaap_AvailableForSaleSecurities / us-gaap_CashAndCashEquivalentsAxis = us-gaap_CertificatesOfDepositMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember
Money Market Funds Held in Trust [Member] |
Concentration Risk [Line Items] |
Marketable securities | 50.1 us-gaap_AvailableForSaleSecurities / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember / us-gaap_InvestmentTypeAxis = us-gaap_MoneyMarketFundsMember
Variable Rate Demand Notes (VRDN) [Member] |
Concentration Risk [Line Items] |
Marketable securities | 21.7 us-gaap_AvailableForSaleSecurities / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember / us-gaap_InvestmentTypeAxis = us-gaap_VariableRateDemandObligationMember
Municipal Bonds [Member] |
Concentration Risk [Line Items] |
Marketable securities | 8.2 us-gaap_AvailableForSaleSecurities / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember
Corporate Bonds [Member] |
Concentration Risk [Line Items] |
Marketable securities | $ 5.7 us-gaap_AvailableForSaleSecurities / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -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 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562 + Details
Name: | us-gaap_AvailableForSaleSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | us-gaap_ConcentrationRiskLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration