Debt (Other Credit Facilities) (Narrative) (Details) $ in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2018 CAD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 CAD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Line of credit facility, letters of credit issued		$ 106.1		$ 157.1
Other Credit Facilities [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity (in dollars)	$ 20.0		$ 20.0
Debt instrument, rationale for classification as long-term debt	Outstanding borrowings that are not renewed are repaid with borrowings under the Credit Facility. Accordingly, the carrying amounts of the Company’s borrowings under its other credit facilities, which are included within other debt obligations in the table above, are classified within long-term debt in the Company’s consolidated balance sheets.
Other Credit Facilities [Member] | Weighted Average [Member]
Debt Instrument [Line Items]
Line of credit facility, interest rate (percentage)	4.50%	4.50%	4.00%	4.00%
Other Credit Facilities [Member] | Canadian Dollars [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity (in dollars)		$ 15.5		$ 15.9
Long-term debt obligations (in dollars)		10.3		$ 10.4
Line of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility, letters of credit issued		17.1
Line of Credit [Member] | Letters of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity (in dollars)		$ 50.0
Line of Credit [Member] | Letters of Credit [Member] | Standby Letters of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility, interest rate (percentage)	0.75%	0.75%