Commitments and Contingencies - Natural Gas Purchases (Details) MMBTU in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) MMBTU
Long-term Purchase Commitment [Line Items]
Commitments to purchase natural gas	$ 4,985,000
Weighted average rate per MMBtu	$ 3.22
MMBtus under fixed-price contracts
Long-term Purchase Commitment [Line Items]
MMBtus under fixed-price contracts (in MMBtus) | MMBTU	1,548