Debt and Debt-Affiliate - Debt (Parenthetical) (Detail) - Jun. 30, 2015 - USD ($) $ in Millions	Total
NRP LP [Member] | 9.125% Senior Notes, with Semi-annual Interest Payments in April and October, Maturing October 2018 [Member]
Debt Instrument [Line Items]
Senior Notes, Face amount	$ 425
Rate of senior notes	9.125%
Opco [Member] | $300 Million Floating Rate Revolving Credit Facility, Due October 2017 [Member]
Debt Instrument [Line Items]
Floating rate revolving credit facility	$ 300
Opco [Member] | $300 Million Floating Rate Revolving Credit Facility, Due August 2016 [Member]
Debt Instrument [Line Items]
Floating rate revolving credit facility	300
Opco [Member] | $200 Million Floating Rate Term Loan, Due January 2016 [Member]
Debt Instrument [Line Items]
Floating rate revolving credit facility	$ 200
Opco [Member] | 4.91% Senior Notes, with Semi-annual Interest Payments in June and December, with Annual Principal Payments in June, Maturing in June 2018 [Member]
Debt Instrument [Line Items]
Rate of senior notes	4.91%
Opco [Member] | 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]
Rate of senior notes	8.38%
Opco [Member] | 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]
Rate of senior notes	5.05%
Opco [Member] | 5.31% Utility Local Improvement Obligation, with Annual Principal and Interest Payments, Maturing in March 2021 [Member]
Debt Instrument [Line Items]
Rate of senior notes	5.31%
Opco [Member] | 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]
Rate of senior notes	5.55%
Opco [Member] | 4.73% Senior Notes, with Semi-annual Interest Payments in June and December, with Annual Principal Payments in December, Maturing in December 2023 [Member]
Debt Instrument [Line Items]
Rate of senior notes	4.73%
Opco [Member] | 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]
Rate of senior notes	5.82%
Opco [Member] | 8.92% 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]
Rate of senior notes	8.92%
Opco [Member] | 5.03% Senior Notes, with Semi-annual Interest Payments in June and December, with Annual Principal Payments in December, Maturing in December 2026 [Member]
Debt Instrument [Line Items]
Rate of senior notes	5.03%
Opco [Member] | 5.18% Senior Notes, with Semi-annual Interest Payments in June and December, with Annual Principal Payments in December, Maturing in December 2026 [Member]
Debt Instrument [Line Items]
Rate of senior notes	5.18%
Senior Notes Issued at 99.007% [Member] | NRP LP [Member] | 9.125% Senior Notes, with Semi-annual Interest Payments in April and October, Maturing October 2018 [Member]
Debt Instrument [Line Items]
Floating rate revolving credit facility	$ 300
Senior Notes issue percentage	99.007%
Senior Notes Issued at 99.5% [Member] | NRP LP [Member] | 9.125% Senior Notes, with Semi-annual Interest Payments in April and October, Maturing October 2018 [Member]
Debt Instrument [Line Items]
Floating rate revolving credit facility	$ 125
Senior Notes issue percentage	99.50%