20. COMMITMENTS AND CONTINGENCIES Commitments  The ARLP Partnership leases buildings and equipment under operating lease agreements that provide for the payment of both minimum and contingent rentals. The ARLP Partnership also has a noncancelable lease with SGP (See Note 19  Related-Party Transactions) and a noncancelable lease for equipment under a capital lease obligation. In 2015, the ARLP Partnership acquired equipment and other assets under operating and capital lease agreements as a result of the Hamilton and Patriot acquisitions (See Note 3  Acquisitions). Future minimum lease payments are as follows: Other Operating Leases Year Ending December 31, Capital Lease Affiliate Others Total (in thousands) 2016 $ 24,138 $ 240 $ 15,107 $ 15,347 2017 23,572 - 11,334 11,334 2018 23,521 - 9,372 9,372 2019 39,716 - 5,371 5,371 2020 690 - 1,239 1,239 Thereafter - - - - Total future minimum lease payments $ 111,637 $ 240 $ 42,423 $ 42,663 Less: amount representing interest (11,723 ) Present value of future minimum lease payments 99,914 Less: current portion (19,764 ) Long-term capital lease obligation $ 80,150 As a result of the idling of the Onton mine, the ARLP Partnership has removed certain rental payments currently under negotiation from the table above (See Note 4  Long-Lived Asset Impairments). Rental expense (including rental expense incurred under operating lease agreements) was $11.7 million, $4.7 million and $5.1 million for the years ended December 31, 2015, 2014 and 2013, respectively. Contractual Commitments  In connection with planned capital projects, the ARLP Partnership has contractual commitments of approximately $28.7 million at December 31, 2015. As of December 31, 2015, the ARLP Partnership had no material commitments to purchase coal from external production sources in 2016. On November 10, 2014, Cavalier Minerals purchased equity interests in AllDale Minerals, an entity created to purchase oil and gas mineral interests in various geographic locations within producing basins in the continenta