Derivatives Derivatives (Notional Amounts of Derivatives) (Details) - Cash Flow Hedging $ in Thousands	Mar. 31, 2018 USD ($) MMBTU	Dec. 31, 2017 USD ($) MMBTU
Natural Gas Contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount | MMBTU	2,160,000	2,480,000
Senior Loans | Interest Rate Swap
Derivative [Line Items]
Derivative, notional amount | $	$ 220,000	$ 220,000