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Stock-Based Compensation, Dividend Restrictions, and Calculations of Earnings Per Share of Common Stock - Schedule of Restricted Stock and Restricted Stock Units (Parenthetical) (Detail)
Dec. 31, 2014
Dec. 31, 2013
Restricted Stock [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number of shares vested but not yet settled 36,110pom_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberVested
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember		  
Performance-Based Restricted Stock Units [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number of shares vested but not yet settled 59,797pom_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberVested
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 31,403pom_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberVested
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
Time-Based Restricted Stock Units Award [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number of shares vested but not yet settled 94,685pom_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberVested
/ us-gaap_AwardTypeAxis
= pom_TimeBasedRestrictedStockUnitsMember		 59,523pom_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberVested
/ us-gaap_AwardTypeAxis
= pom_TimeBasedRestrictedStockUnitsMember