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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
Certificate of Deposits [Member]
  
Concentration Risk [Line Items]  
Marketable securities $ 137.6us-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 50.1us-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 21.7us-gaap_AvailableForSaleSecurities
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_VariableRateDemandObligationMember
Municipal Bonds [Member]
  
Concentration Risk [Line Items]  
Marketable securities 8.2us-gaap_AvailableForSaleSecurities
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_MunicipalBondsMember
Corporate Bonds [Member]
  
Concentration Risk [Line Items]  
Marketable securities $ 5.7us-gaap_AvailableForSaleSecurities
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateDebtSecuritiesMember