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Debt and Debt - Affiliate - Long-Term Debt (Detail) - USD ($)
Dec. 31, 2015
Jun. 30, 2015
Dec. 31, 2014
Oct. 31, 2014
Sep. 30, 2013
Debt Instrument [Line Items]          
Total debt $ 1,384,996,000   $ 1,475,223,000    
Less - current portion of long term debt (80,983,000)   (80,983,000)    
Long-term debt, net $ 1,304,013,000   1,394,240,000    
Rate of senior notes 9.125%        
NRP LP          
Debt Instrument [Line Items]          
Floating rate revolving credit facility       $ 125,000,000 $ 300,000,000.0
Senior Note issue percentage       99.50% 99.007%
Rate of senior notes       9.125% 9.125%
NRP LP | 9.125% senior notes, with semi-annual interest payments in April and October, maturing October 2018          
Debt Instrument [Line Items]          
Total debt $ 422,923,000   422,167,000    
Debt Instrument, face Amount $ 425,000,000        
Rate of senior notes 9.125%        
Opco | $300 million floating rate revolving credit facility, due October 2017          
Debt Instrument [Line Items]          
Total debt $ 290,000,000   0    
Floating rate revolving credit facility 300,000,000 $ 300,000,000.0      
Opco | $300 million floating rate revolving credit facility, due August 2016          
Debt Instrument [Line Items]          
Total debt 0   200,000,000    
Floating rate revolving credit facility 300,000,000 $ 300,000,000.0      
Opco | $200 million floating rate term loan, due January 2016          
Debt Instrument [Line Items]          
Total debt 0   75,000,000    
Floating rate revolving credit facility 200,000,000        
Opco | 4.91% senior notes, with semi-annual interest payments in June and December, with annual principal payments in June, maturing in June 2018          
Debt Instrument [Line Items]          
Total debt $ 13,850,000   18,467,000    
Rate of senior notes 4.91%        
Opco | 8.38% senior notes, with semi-annual interest payments in March and September, with annual principal payments in March, maturing in March 2019 [Member]          
Debt Instrument [Line Items]          
Total debt $ 85,714,000   107,143,000    
Rate of senior notes 8.38%        
Opco | 5.05% senior notes, with semi-annual interest payments in January and July, with annual principal payments in July, maturing in July 2020 [Member]          
Debt Instrument [Line Items]          
Total debt $ 38,462,000   46,154,000    
Rate of senior notes 5.05%        
Opco | 5.31% utility local improvement obligation, with annual principal and interest payments, maturing in March 2021 [Member]          
Debt Instrument [Line Items]          
Total debt $ 1,153,000   1,345,000    
Rate of senior notes 5.31%        
Opco | 5.55% senior notes, with semi-annual interest payments in June and December, with annual principal payments in June, maturing in June 2023 [Member]          
Debt Instrument [Line Items]          
Total debt $ 21,600,000   24,300,000    
Rate of senior notes 5.55%        
Opco | 4.73% senior notes, with semi-annual interest payments in June and December, with scheduled principal payments beginning December 2014, maturing in December 2023 [Member]          
Debt Instrument [Line Items]          
Total debt $ 60,000,000   67,500,000    
Rate of senior notes 4.73%        
Opco | 5.82% senior notes, with semi-annual interest payments in March and September, with annual principal payments in March, maturing in March 2024 [Member]          
Debt Instrument [Line Items]          
Total debt $ 135,000,000   150,000,000    
Rate of senior notes 5.82%        
Opco | 8.92% senior notes, with semi-annual interest payments in March and September, with scheduled principal payments beginning March 2014, maturing in March 2024 [Member]          
Debt Instrument [Line Items]          
Total debt $ 40,909,000   45,455,000    
Rate of senior notes 8.92%        
Opco | 5.03% senior notes, with semi-annual interest payments in June and December, with scheduled principal payments beginning December 2014, maturing in December 2026 [Member]          
Debt Instrument [Line Items]          
Total debt $ 148,077,000   161,538,000    
Rate of senior notes 5.03%        
Opco | 5.18% senior notes, with semi-annual interest payments in June and December, with scheduled principal payments beginning December 2014, maturing in December 2026 [Member]          
Debt Instrument [Line Items]          
Total debt $ 42,308,000   46,154,000    
Rate of senior notes 5.18%        
NRP Oil and Gas | Reserve Based Revolving Credit Facility Due 2019 [Member]          
Debt Instrument [Line Items]          
Total debt $ 85,000,000   $ 110,000,000    
Senior Notes Offering Price Two [Member] | NRP LP | 9.125% senior notes, with semi-annual interest payments in April and October, maturing October 2018          
Debt Instrument [Line Items]          
Floating rate revolving credit facility $ 125,000,000        
Senior Note issue percentage 99.50%        
Senior Notes Offering Price One [Member] | NRP LP | 9.125% senior notes, with semi-annual interest payments in April and October, maturing October 2018          
Debt Instrument [Line Items]          
Floating rate revolving credit facility $ 300,000,000        
Senior Note issue percentage 99.007%