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Contractual Coupon Interest Expense (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Debt Instrument [Line Items]    
Contractual coupon interest expense $ 1,266us-gaap_InterestExpenseDebt $ 1,266us-gaap_InterestExpenseDebt
Accretion of discount on the liability component $ 1,037us-gaap_AccretionExpense $ 968us-gaap_AccretionExpense