Financing (Unsecured Revolving Credit Facility) (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended		12 Months Ended
	Sep. 09, 2011	Jan. 14, 2011	Dec. 31, 2012	Dec. 31, 2011
Letters of credit			$ 57.3
Unsecured Debt [Member] | Line of Credit Facility [Member]
Credit agreement inception date		Jan. 14, 2011
Number of years in credit facility term		5
Current maximum borrowing capacity under credit facility	660	750
Line of credit facility expiration date	Sep. 09, 2016	Jan. 14, 2016
Credit agreement amendment date	Sep. 09, 2011
One-time charge related to the modification of the credit agreement				0.3
Maximum aggregate increase to credit facility	200
Outstanding borrowings under credit facility			0	0
Line of credit facility fee percentage			0.175%
Covenant description for debt instrument		The Credit Agreement contains certain covenants, which include limitations on indebtedness, a minimum fixed charge coverage ratio of 2.00 times through December 31, 2012; 2.25 times thereafter through December 31, 2014; and 2.50 times thereafter through maturity; and a maximum adjusted consolidated leverage ratio of 3.00 times through maturity. The consolidated leverage ratio includes a calculation of adjusted earnings before interest, taxes, depreciation, amortization, rent and stock based compensation expense to adjusted debt. Adjusted debt includes outstanding debt, outstanding letters of credit, six-times rent expense and excludes any premium or discount recorded in conjunction with the issuance of long-term debt. In the event that the Company should default on any covenant contained within the Credit Agreement, certain actions may be taken, including, but not limited to, possible termination of credit extensions, immediate payment of outstanding principal amount plus accrued interest and other amounts payable under the Credit Agreement and litigation from lenders.
Line of credit facility covenant compliance			As of December 31, 2012, the Company remained in compliance with all covenants under the Credit Agreement.
Unsecured Debt [Member] | Line of Credit Facility [Member] | Letter of Credit [Member]
Letters of credit			57.3	59.9
Line of credit facility sublimit		200
Unsecured Debt [Member] | Line of Credit Facility [Member] | Swing Line Revolver [Member]
Line of credit facility sublimit		$ 75
Unsecured Debt [Member] | Line of Credit Facility [Member] | Euro Dollar Rate [Member]
Line of credit current interest rate			1.20%
Unsecured Debt [Member] | Line of Credit Facility [Member] | Base Rate [Member]
Line of credit current interest rate			0.20%
Unsecured Debt [Member] | Line of Credit Facility [Member] | Through December 2012 [Member]
Minimum debt instrument consolidated fixed charge coverage ratio covenant		2.00%
Unsecured Debt [Member] | Line of Credit Facility [Member] | Through December 2014 [Member]
Minimum debt instrument consolidated fixed charge coverage ratio covenant		2.25%
Unsecured Debt [Member] | Line of Credit Facility [Member] | Through Maturity [Member]
Minimum debt instrument consolidated fixed charge coverage ratio covenant		2.50%
Maximum debt instrument consolidated leverage ratio covenant		3.00%