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Future Lease Payment Under Terms of Capacity Purchase Agreement (Detail)
$ in Billions
Dec. 31, 2017
USD ($)
Long-term Purchase Commitment [Line Items]  
2018 $ 2.0
2019 1.8
2020 1.6
2021 1.5
2022 1.4
After 2022 3.2
Contractual Obligation, Total $ 11.5