Debt, Credit Facilities (Details)	9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2015 CAD	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 CAD	Dec. 31, 2014 USD ($)
Other Letter of Credit [Member]
Line of Credit Facility
Amount outstanding			$ 93,000,000			$ 80,000,000
Credit Facilities [Member] | Letter of Credit [Member]
Line of Credit Facility
Line of credit facility, maximum borrowing capacity			125,000,000
Line of credit facility, expiration date	Jun. 30, 2016
Amount outstanding			20,000,000			56,000,000
Credit Facilities [Member] | Committed Letter of Credit - Expired [Member]
Line of Credit Facility
Line of credit facility, maximum borrowing capacity				$ 300,000,000
Credit Facilities [Member] | Committed Letter of Credit - In Effect [Member]
Line of Credit Facility
Line of credit facility, maximum borrowing capacity			125,000,000	$ 250,000,000
Credit Facilities [Member] | U S Revolver [Member]
Line of Credit Facility
Line of credit facility, maximum borrowing capacity			3,000,000,000
Line of credit facility, expiration date	Nov. 30, 2018
Amount outstanding			0			0
Credit Facilities (Textual)
Line of credit facility, accordion feature, higher borrowing capacity option			$ 4,500,000,000
Credit Facilities [Member] | U S Revolver [Member] | Maximum [Member]
Credit Facilities (Textual)
Debt to capitalization ratios as per terms of revolver		0.60	0.60
Credit Facilities [Member] | U S Revolver Letter of Credit [Member]
Line of Credit Facility
Amount outstanding			$ 54,000,000			54,000,000
Credit Facilities [Member] | VLP Revolver [Member]
Line of Credit Facility
Line of credit facility, maximum borrowing capacity			300,000,000
Line of credit facility, expiration date	Dec. 31, 2018
Amount outstanding			175,000,000			0
Credit Facilities (Textual)
Line of credit facility, interest rate during period	1.50%
Borrowings from long-term lines of credit	$ 200,000,000
Repayments of long-term lines of credit	$ 25,000,000
Credit Facilities [Member] | VLP Revolver Letter of Credit [Member]
Line of Credit Facility
Amount outstanding			$ 0			$ 0
Credit Facilities [Member] | Canadian Revolver [Member]
Line of Credit Facility
Line of credit facility, maximum borrowing capacity | CAD		CAD 50,000,000
Line of credit facility, expiration date	Nov. 30, 2015
Amount outstanding | CAD		0			CAD 0
Credit Facilities [Member] | Canadian Revolver Letter of Credit [Member]
Line of Credit Facility
Amount outstanding | CAD		CAD 10,000,000			CAD 10,000,000