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Condensed Consolidating Financial Statements (Details Textual)
3 Months Ended
Mar. 31, 2015
Condensed Financial Statements, Captions [Line Items]  
Ownership percentage held by holding company 100.00%npci_OwnershipPercentageHeldByHoldingCompany
Ownership Percentage of Co Issuer held by Company 100.00%npci_OwnershipPercentageOfCoIssuerHeldByCompany
Senior Notes  
Condensed Financial Statements, Captions [Line Items]  
Interest on senior notes 10.50%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember