Basis of Presentation - Additional Information (Detail)						1 Months Ended		3 Months Ended	9 Months Ended		21 Months Ended
	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Feb. 18, 2016 shares	Oct. 31, 2014 USD ($)	Sep. 30, 2013 USD ($)	Mar. 31, 2016 USD ($)	Sep. 30, 2016 USD ($) shares	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) shares	Feb. 17, 2016 shares	Dec. 31, 2015 USD ($) shares
Schedule Of Significant Accounting Policies [Line Items]
Deferred issuance costs									$ 13,806,000		$ 13,806,000		$ 14,040,000
Total debt									$ 1,200,581,000		1,200,581,000		$ 1,370,956,000
Repayments of Debt											$ 262,200,000
Common units outstanding (in shares) | shares					12,200,000				12,232,006		12,232,006	122,300,000	12,232,006
Immaterial error correction								$ 8,000,000
Proceeds from Sale of Property, Plant, and Equipment									$ 9,800,000	$ 3,700,000	$ 192,000,000
NRP LP
Schedule Of Significant Accounting Policies [Line Items]
Rate of senior notes (percent)							9.125%
Repayment of principal amount						$ 122,600,000
NRP LP | 9.125% senior notes, with semi-annual interest payments in April and October, maturing October 2018
Schedule Of Significant Accounting Policies [Line Items]
Debt Instrument, face amount									$ 425,000,000		$ 425,000,000
Rate of senior notes (percent)									9.125%		9.125%
Opco
Schedule Of Significant Accounting Policies [Line Items]
Repayment of principal amount							$ 289,000,000
Opco | Floating Rate Revolving Credit Facility Due June Two Thousand Eighteen [Member]
Schedule Of Significant Accounting Policies [Line Items]
Total debt									$ 260,000,000		$ 260,000,000
Opco | Floating Rate Revolving Credit Facility Due June Two Thousand Eighteen [Member] | Scenario, Forecast [Member]
Schedule Of Significant Accounting Policies [Line Items]
Repayment of principal amount			$ 30,000,000	$ 50,000,000
Opco | Floating Rate Revolving Credit Facility Due October Two Thousand Seventeen [Member] | Scenario, Forecast [Member]
Schedule Of Significant Accounting Policies [Line Items]
Repayment of principal amount	$ 150,000,000	$ 30,000,000	$ 80,800,000
Opco | Senior Notes
Schedule Of Significant Accounting Policies [Line Items]
Total debt									529,900,000		$ 529,900,000
Repayments of Debt									$ 56,000,000	$ 56,000,000
Revolving Credit Facility | Opco
Schedule Of Significant Accounting Policies [Line Items]
Ratio of Indebtedness									2.95
Common Unitholders
Schedule Of Significant Accounting Policies [Line Items]
Reverse split ratio, common units					0.1
Maximum | Opco
Schedule Of Significant Accounting Policies [Line Items]
Ratio of Indebtedness									4.0
Maximum | Revolving Credit Facility | Opco
Schedule Of Significant Accounting Policies [Line Items]
Ratio of Indebtedness									4.0
Short-term Debt | Accounting Standards Update 2015-03
Schedule Of Significant Accounting Policies [Line Items]
Deferred issuance costs													$ 200,000
Other Current Assets | Accounting Standards Update 2015-03
Schedule Of Significant Accounting Policies [Line Items]
Deferred issuance costs													200,000
Other Noncurrent Assets | Accounting Standards Update 2015-03
Schedule Of Significant Accounting Policies [Line Items]
Deferred issuance costs													13,800,000
Long-term Debt | Accounting Standards Update 2015-03
Schedule Of Significant Accounting Policies [Line Items]
Deferred issuance costs													$ 13,800,000