v3.5.0.2
Commitments and Contingencies - Natural Gas Purchases (Details)
MMBTU in Thousands
9 Months Ended
Sep. 30, 2016
USD ($)
MMBTU
Long-term Purchase Commitment [Line Items]  
Commitments to purchase natural gas $ 1,768,000
Weighted average rate per MMBtu $ 2.85
MMBtus under fixed-price contracts  
Long-term Purchase Commitment [Line Items]  
MMBtus under fixed-price contracts | MMBTU 620