Derivatives Notional Amounts of Derivatives (Details) CAD in Thousands, $ in Thousands	Dec. 31, 2017 USD ($) MMBTU	Dec. 31, 2017 CAD MMBTU	Dec. 31, 2016 USD ($) MMBTU	Dec. 31, 2016 CAD MMBTU
Cash Flow Hedging | Natural Gas Contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount | MMBTU	2,480,000	2,480,000	2,590,000	2,590,000
Not Designated as Hedging Instrument | Currency Contracts
Derivative [Line Items]
Derivative, notional amount | CAD		CAD 0		CAD 0
Senior Loans | Cash Flow Hedging | Interest Rate Swap
Derivative [Line Items]
Derivative, notional amount | $	$ 220,000		$ 220,000