Debt, Net - Debt (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Principal balance	$ 498,215	$ 524,056
Net unamortized debt issuance costs	(6,742)	(7,858)
Total debt, net	491,473	516,198
Less - current portion of long term debt	(39,072)	(45,776)
Total long-term debt, net	452,401	470,422
NRP LP \| 9.125% senior notes, with semi-annual interest payments in June and December, maturing June 2025
Debt Instrument [Line Items]
Principal balance	$ 300,000	300,000
Rate of senior notes	9.125%
Opco \| Floating Rate Revolving Credit Facility Due April Two Thousand Twenty Three
Debt Instrument [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	$ 100,000
Principal balance	0	0
Opco \| 5.05% senior notes, with semi-annual interest payments in January and July, with annual principal payments in July, maturing in July 2020
Debt Instrument [Line Items]
Principal balance	$ 0	6,780
Rate of senior notes	5.05%
Opco \| 5.55% senior notes, with semi-annual interest payments in June and December, with annual principal payments in June, maturing in June 2023
Debt Instrument [Line Items]
Principal balance	$ 7,094	9,458
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
Debt Instrument [Line Items]
Principal balance	$ 24,016	24,016
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
Debt Instrument [Line Items]
Principal balance	$ 50,738	63,423
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
Debt Instrument [Line Items]
Principal balance	$ 16,047	20,059
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
Debt Instrument [Line Items]
Principal balance	$ 79,945	79,945
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
Debt Instrument [Line Items]
Principal balance	$ 20,375	20,375
Rate of senior notes	5.18%
Opco \| Senior Notes
Debt Instrument [Line Items]
Principal balance	$ 198,215	$ 224,056

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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