Long-Term Debt (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Long-term Debt Types [Abstract]
Annual Facility Fee	0.225%
Debt Required Payments Start Date	Jan. 01, 2015
Proceeds from Issuance of Debt	$ 0	$ 0	$ 100.0
Bonds Face Amount	43.0
Undrawn Letters of Credit	137.0
October 2014 Multi-currency Facility [Member]
Long-term Debt Types [Abstract]
Debt Due Date	Oct. 01, 2014
Credit Facility Availble Borrowing Credit	30.0
Revolving Facility [Member]
Long-term Debt Types [Abstract]
Unsecured notes outstanding	120.6	107.2
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	359.0
Amended Revolving Facility [Member] | Libor [Member]
Long-term Debt Types [Abstract]
Margin on Base Rate	1.40%
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	73.0
Credit Facility Availble Borrowing Credit	46.0
Undrawn Letters of Credit	21.0
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	23.0
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	179.0
Credit Facility Availble Borrowing Credit	70.0
Undrawn Letters of Credit	109.0
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, 2016
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, 2015
Credit Facility Availble Borrowing Credit	40.0
Unsecured Bilateral Commited Credit Facilities [Member]
Long-term Debt Types [Abstract]
Credit Facility Borrowing Capacity	24.0
Debt Due Date	Apr. 01, 2014
Credit Facility Availble Borrowing Credit	14.0
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	$ 7.1