Derivatives And Risk Management Derivatives and Risk Management (Foreign Currency Exchange Contracts) (Details) $ in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018 CAD ($) derivative_contracts	Mar. 31, 2018 USD ($) derivative_contracts	Dec. 31, 2017 CAD ($) derivative_contracts	Dec. 31, 2017 USD ($) derivative_contracts
Foreign Currency Fair Value Hedge Derivative [Line Items]
Number Of Days Canadian Currency Prices Are Settled With U.S. Dollars	60 days
Number of Foreign Currency Derivatives Held | derivative_contracts	23	23	18	18
United States of America, Dollars | Foreign Exchange Contract [Member]
Foreign Currency Fair Value Hedge Derivative [Line Items]
Derivative, Notional Amount		$ 5,312		$ 2,552
Canada, Dollars | Foreign Exchange Contract [Member]
Foreign Currency Fair Value Hedge Derivative [Line Items]
Derivative, Notional Amount	$ 6,866		$ 3,241