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Debt - 2006 Senior Unsecured Notes - Additional Information (Detail) (2006 Senior Unsecured Notes [Member], USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2006
Debt Instrument [Line Items]    
Senior unsecured notes, value of offering   $ 125.0us-gaap_UnsecuredDebt
Credit facilities, periodic payments of interest $ 65.0us-gaap_DebtInstrumentPeriodicPaymentInterest  
Senior notes frequency of periodic payment Quarterly  
Unsecured senior notes, year of maturity 2016-07  
London Interbank Offered Rate [Member]
   
Debt Instrument [Line Items]    
Debt instrument, basis spread of interest rate 2.30%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember