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Stock Options - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 1 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Jan. 17, 2013
Dec. 31, 2012
Class of Stock [Line Items]        
Allocated Share-based Compensation Expense $ 159us-gaap_AllocatedShareBasedCompensationExpense $ 88us-gaap_AllocatedShareBasedCompensationExpense    
Employee Stock Option [Member]        
Class of Stock [Line Items]        
Share Based Compensation Arrangement By Share Based Payment Award Percentage Of Award Vested After One Year     50.00%posc_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfAwardVestedAfterOneYear
/ us-gaap_DeferredCompensationArrangementWithIndividualShareBasedPaymentsByTypeOfDeferredCompensationAxis
= us-gaap_EmployeeStockOptionMember		  
Share Based Compensation Arrangement By Share Based Payment Award Options Vested Percentage       50.00%posc_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedPercentage
/ us-gaap_DeferredCompensationArrangementWithIndividualShareBasedPaymentsByTypeOfDeferredCompensationAxis
= us-gaap_EmployeeStockOptionMember