Fair value Measurements - Additional Information (Detail) $ in Millions	Mar. 31, 2015 USD ($)
NRP LP Senior Notes [Member]
Fair Value Measurements Disclosure [Line Items]
Estimated fair value	$ 378.3
NRP LP Senior Notes [Member] | Scenario, Previously Reported [Member]
Fair Value Measurements Disclosure [Line Items]
Estimated fair value	417.0
Opco Senior Notes and Utility Local Improvement Obligation [Member]
Fair Value Measurements Disclosure [Line Items]
Estimated fair value	557.9
Opco Senior Notes and Utility Local Improvement Obligation [Member] | Scenario, Previously Reported [Member]
Fair Value Measurements Disclosure [Line Items]
Estimated fair value	$ 629.5