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Option Granted by Company (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of options granted 60,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod 60,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod 1,503,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod
Exercise price $ 8.05us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice $ 7.50us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice $ 6.03us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
January 2012 Grant      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of options granted 600,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod
/ us-gaap_AwardDateAxis
= ntp_GrantDateOneMember		 [1]    
Vesting period 100% vested in April 2013 [1]    
Exercise price $ 6.66us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardDateAxis
= ntp_GrantDateOneMember		 [1]    
Exercisable period, start date Apr. 01, 2013 [1]    
Exercisable period, end date Apr. 30, 2016 [1]    
Weighted average remaining contractual life in months 16 months [1]    
April 2012 Grant      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of options granted 831,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod
/ us-gaap_AwardDateAxis
= ntp_GrantDateTwoMember		    
Vesting period 50% vested in January 2013, 40% vested in 2014 and 10% will vest after 2014    
Exercise price $ 5.63us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardDateAxis
= ntp_GrantDateTwoMember		    
Exercisable period, start date Jan. 01, 2013    
Exercisable period, end date Apr. 26, 2015    
Weighted average remaining contractual life in months 3 months 26 days    
June 2012 Grant      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of options granted 60,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod
/ us-gaap_AwardDateAxis
= ntp_GrantDateThreeMember		    
Vesting period 100% vested at date of grant    
Exercise price $ 5.34us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardDateAxis
= ntp_GrantDateThreeMember		    
Exercisable period, start date Jun. 06, 2012    
Exercisable period, end date Jun. 05, 2015    
Weighted average remaining contractual life in months 5 months 5 days    
2013 Grant      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of options granted 60,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod
/ us-gaap_AwardDateAxis
= ntp_GrantDateFourMember		    
Vesting period 100% vested at date of grant    
Exercise price $ 7.50us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardDateAxis
= ntp_GrantDateFourMember		    
Exercisable period, start date May 31, 2013    
Exercisable period, end date May 31, 2016    
Weighted average remaining contractual life in months 17 months    
2014 Grant      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of options granted 60,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod
/ us-gaap_AwardDateAxis
= ntp_GrantDateFiveMember		    
Vesting period 100% vested at date of grant    
Exercise price $ 8.05us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardDateAxis
= ntp_GrantDateFiveMember		    
Exercisable period, start date Jun. 06, 2014    
Exercisable period, end date Jun. 05, 2017    
Weighted average remaining contractual life in months 29 months 5 days    
[1] Exercisable period modified in 2013