Debt, Credit Facilities (Details)		1 Months Ended	6 Months Ended
	Jul. 01, 2015 USD ($)	Jul. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2015 CAD	Jun. 30, 2015 USD ($)	Dec. 31, 2014 CAD	Dec. 31, 2014 USD ($)
Other Letter of Credit [Member]
Line of Credit Facility
Amount outstanding					$ 263,000,000		$ 80,000,000
Credit Facilities [Member] | Letter of Credit [Member]
Line of Credit Facility
Line of credit facility, maximum borrowing capacity					250,000,000
Line of credit facility, expiration date			Jun. 30, 2015
Amount outstanding					0		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					250,000,000
Credit Facilities [Member] | Committed Letter of Credit - In Effect [Member] | Subsequent Event [Member]
Line of Credit Facility
Line of credit facility, maximum borrowing capacity		$ 125,000,000
Line of credit facility, expiration date		Jun. 30, 2016
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					$ 63,000,000		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					200,000,000		0
Credit Facilities (Textual)
Borrowings from long-term lines of credit			$ 200,000,000
Line of credit facility, interest rate during period			1.4375%
Credit Facilities [Member] | VLP Revolver [Member] | Subsequent Event [Member]
Credit Facilities (Textual)
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