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Future Lease Payment Under Terms of Capacity Purchase Agreement (Detail)
$ in Billions
Mar. 31, 2017
USD ($)
Long-term Purchase Commitment [Line Items]  
Last nine months of 2017 $ 1.3
2018 1.9
2019 1.4
2020 1.1
2021 1.0
After 2021 4.0
Contractual Obligation, Total $ 10.7