Assets and Liabilities Measured at Fair Value (Carrying Amounts and Estimated Fair Values of Financial Instruments) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial assets (liabilities), Carrying value
Canadian government securities with maturities greater than 90 days at the date of acquisition		$ 111,542
Current and long-term debt	$ (2,916,422)	(2,992,567)
Financial assets (liabilities), Fair value
Canadian government securities with maturities greater than 90 days at the date of acquisition		111,331
Current and long-term debt	$ (2,993,003)	$ (2,951,992)