Derivatives (Narrative) (Details) - Designated as hedging instrument gal in Millions, CAD in Millions, $ in Millions		12 Months Ended
	Dec. 31, 2017 gal	Dec. 31, 2017 USD ($) gal	Dec. 31, 2017 CAD gal	Dec. 31, 2016 USD ($) gal	Dec. 31, 2015 USD ($) gal
Diesel swap
Derivative [Line Items]
Hedging activities net cash flow impact | $		$ (17)		$ 29	$ 35
Derivative, amount of hedged item, volume, used (in gallons)		6.7	6.7	10.1	10.6
Cash Flow Hedging | Diesel swap
Derivative [Line Items]
Derivative, nonmonetary notional amount (in gallons)	4.7
Intersegment Eliminations | Forward contracts
Derivative [Line Items]
Derivative purchases of underlying currency (in Canadian Dollars) | CAD			CAD 1,070