Including the current and noncurrent portions, the carrying value of convertible subordinated debt, as of the balance sheet date, initially scheduled to be repaid after one year or beyond the normal operating cycle if longer. This form of debt can be exchanged for a specified amount of another security, typically the entity's common stock, at the option of the issuer or the holder, and places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets.

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Derivatives (Realized and Unrealized Gains (Losses) with Respect to Cash Flow Hedges) (Details) $ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017 CAD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 CAD ($)	Dec. 31, 2016 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives	$ 157		$ (258)
Other Derivatives Hedge [Member] \| Interest Rate Swap [Member] \| Line of Credit [Member] \| Brunswick Pipeline [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Line of Credit Facility, Expiration Date	Dec. 31, 2019	Dec. 31, 2019
Convertible Subordinated Debt	$ 250
Designated as Hedging Instrument [Member] \| Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	$ 2		$ 2
Designated as Hedging Instrument [Member] \| Interest Rate Swap [Member] \| Other Operating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives		$ 0		$ 0
Designated as Hedging Instrument [Member] \| Interest Rate Swap [Member] \| Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives		2		2
Designated as Hedging Instrument [Member] \| Foreign Exchange Option [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net		0		(87)
Designated as Hedging Instrument [Member] \| Foreign Exchange Option [Member] \| Other Operating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives		0		(87)
Designated as Hedging Instrument [Member] \| Foreign Exchange Option [Member] \| Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives		$ 0		$ 0