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Future Lease Payment Under Terms of Capacity Purchase Agreement (Detail)
$ in Billions
Sep. 30, 2017
USD ($)
Long-term Purchase Commitment [Line Items]  
Last three months of 2017 $ 0.5
2018 2.0
2019 1.8
2020 1.6
2021 1.5
After 2021 4.6
Contractual Obligation, Total $ 12.0