Long-Term Debt (Unsecured Revolving Credit Facility) (Details) (Unsecured Debt [Member], Line of Credit Facility [Member], USD $)	0 Months Ended		3 Months Ended	9 Months Ended
	Sep. 09, 2011	Jan. 14, 2011	Sep. 30, 2012	Sep. 30, 2012	Dec. 31, 2011
Credit agreement inception date		Jan. 14, 2011
Number of years in credit facility term	5
Current maximum borrowing capacity under credit facility	$ 660,000,000
Credit agreement amendment date	Sep. 09, 2011
Line of credit facility expiration date	Sep. 09, 2016
Maximum aggregate increase to credit facility	200,000,000
Letters of credit			57,600,000	57,600,000	59,900,000
Outstanding borrowings under credit facility			0	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, without limitation, outstanding debt, outstanding letters of credit and 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 acceleration of outstanding principal amounts plus accrued interest and other amounts payable under the Credit Agreement and litigation from lenders.
Line of credit facility covenant compliance				As of September 30, 2012, the Company remained in compliance with all covenants under the Credit Agreement.
Letter of Credit [Member]
Line of credit facility sublimit	200,000,000
Swing Line Revolver [Member]
Line of credit facility sublimit	$ 75,000,000
Euro Dollar Rate [Member]
Line of credit current interest rate			1.20%	1.20%
Base Rate [Member]
Line of credit current interest rate			0.20%	0.20%
Through December 2012 [Member]
Minimum debt instrument consolidated fixed charge coverage ratio covenant	2.00
Through December 2014 [Member]
Minimum debt instrument consolidated fixed charge coverage ratio covenant	2.25
Through Maturity [Member]
Minimum debt instrument consolidated fixed charge coverage ratio covenant	2.50
Maximum debt instrument consolidated leverage ratio covenant	3.00