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Commitments and Contingencies - Additional Information (Detail)
$ in Millions
9 Months Ended
Sep. 30, 2017
USD ($)
Commitments And Contingencies Disclosure [Abstract]  
Transportation commitments future minimum transportation fees due to nine months change $ 402.0
Period of transportation commitments future minimum transportation fees due to nine months change 22 years
Transportation commitments future minimum transportation fees due to nine months change, end year 2038