Long-term Debt (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Long-term Debt Types [Abstract]
Annual Facility Fee	0.175%
Debt Required Payments Start Date	Jan. 01, 2015
Proceeds from Issuance of Debt	$ 0	$ 100.0	$ 0
Bonds Face Amount	43.0
Undrawn Letters of Credit	138.4
Revolving Facility [Member]
Long-term Debt Types [Abstract]
Unsecured notes outstanding	107.2	110.0
Amended Revolving Facility [Member]
Long-term Debt Types [Abstract]
Debt Issuance or Start Date	Jan. 06, 2012
Credit Facility Borrowing Capacity	480.0
Debt Due Date	Jan. 01, 2017
Credit Facility Availble Borrowing Credit	373.0
Amended Revolving Facility [Member] | Libor [Member]
Long-term Debt Types [Abstract]
Margin on Base Rate	1.20%
Amended Revolving Facility [Member] | Minimum [Member]
Long-term Debt Types [Abstract]
Annual Facility Fee	0.10%
Amended Revolving Facility [Member] | Minimum [Member] | Libor [Member]
Long-term Debt Types [Abstract]
Margin on Base Rate	0.90%
Amended Revolving Facility [Member] | Minimum [Member] | Alternate Base Rate [Member]
Long-term Debt Types [Abstract]
Margin on Base Rate	0.00%
Amended Revolving Facility [Member] | Maximum [Member]
Long-term Debt Types [Abstract]
Annual Facility Fee	0.30%
Amended Revolving Facility [Member] | Maximum [Member] | Libor [Member]
Long-term Debt Types [Abstract]
Margin on Base Rate	1.575%
Amended Revolving Facility [Member] | Maximum [Member] | Alternate Base Rate [Member]
Long-term Debt Types [Abstract]
Margin on Base Rate	0.575%
Private Placement Notes [Member]
Long-term Debt Types [Abstract]
Debt Due Date	Jan. 01, 2021
Proceeds from Issuance of Debt	100.0
Private Placement Notes [Member] | Private Placement Notes Seriesa [Member]
Long-term Debt Types [Abstract]
Unsecured notes outstanding	50.0
Private Placement Notes [Member] | Private Placement Notes Series B [Member]
Long-term Debt Types [Abstract]
Unsecured notes outstanding	50.0
Unsecured Multi-currency Debt Facilities [Member]
Long-term Debt Types [Abstract]
Credit Facility Borrowing Capacity	70.0
Credit Facility Availble Borrowing Credit	37.0
Undrawn Letters of Credit	21.7
Unsecured Multi-currency Debt Facilities [Member] | October 2014 Multi-currency Facility [Member]
Long-term Debt Types [Abstract]
Debt Due Date	Oct. 01, 2014
Credit Facility Availble Borrowing Credit	30.0
Unsecured Multi-currency Debt Facilities [Member] | December 2012 Multi-currency Facility [Member]
Long-term Debt Types [Abstract]
Debt Due Date	Dec. 01, 2015
Credit Facility Availble Borrowing Credit	20.0
Unsecured Multi-currency Debt Facilities [Member] | May 2014 Multi-currency Facility [Member]
Long-term Debt Types [Abstract]
Debt Due Date	May 01, 2014
Credit Facility Availble Borrowing Credit	20.0
Unsecured Multi-currency Debt Facilities [Member] | Minimum [Member]
Long-term Debt Types [Abstract]
Margin on Base Rate	0.90%
Unsecured Multi-currency Debt Facilities [Member] | Maximum [Member]
Long-term Debt Types [Abstract]
Margin on Base Rate	2.125%
Unsecured Letter of Credit Facilities [Member]
Long-term Debt Types [Abstract]
Credit Facility Borrowing Capacity	164.0
Credit Facility Availble Borrowing Credit	56.0
Undrawn Letters of Credit	108.3
Unsecured Letter of Credit Facilities [Member] | Fifty Four Million Unsecured Letter of Credit Facility [Member]
Long-term Debt Types [Abstract]
Debt Due Date	Dec. 01, 2014
Credit Facility Availble Borrowing Credit	54.0
Unsecured Letter of Credit Facilities [Member] | Eighty Five Million Unsecured Letter of Credit Facility [Member]
Long-term Debt Types [Abstract]
Debt Due Date	Jun. 01, 2015
Credit Facility Availble Borrowing Credit	85.0
Unsecured Letter of Credit Facilities [Member] | Twenty Four Million Unsecured Letter of Credit Facility [Member]
Long-term Debt Types [Abstract]
Debt Due Date	Dec. 01, 2014
Credit Facility Availble Borrowing Credit	25.0
Unsecured Bilateral Commited Credit Facilities [Member]
Long-term Debt Types [Abstract]
Credit Facility Borrowing Capacity	20.0
Debt Due Date	Apr. 01, 2014
Credit Facility Availble Borrowing Credit	16.0
Unsecured Bilateral Commited Credit Facilities [Member] | Minimum [Member]
Long-term Debt Types [Abstract]
Margin on Base Rate	1.20%
Unsecured Bilateral Commited Credit Facilities [Member] | Maximum [Member]
Long-term Debt Types [Abstract]
Margin on Base Rate	1.575%
Dominion Terminal Associates Bonds [Member]
Long-term Debt Types [Abstract]
Debt Due Date	Jan. 01, 2033
Other Credit Facilities [Member]
Long-term Debt Types [Abstract]
Undrawn Letters of Credit	$ 8.4