Indebtedness And Borrowing Facilities (2019 Notes) (Details) (USD $)	12 Months Ended		0 Months Ended			0 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	May 03, 2011 Eight Year 6.75% Unsecured Note Payable Due In 2019 [Member]	Dec. 31, 2012 Eight Year 6.75% Unsecured Note Payable Due In 2019 [Member]	Dec. 31, 2011 Eight Year 6.75% Unsecured Note Payable Due In 2019 [Member]	Jan. 04, 2011 2016 Credit Facility [Member]	Dec. 31, 2012 2016 Credit Facility [Member]
Debt Instrument [Line Items]
Debt instrument, issuance date			May 03, 2011
Debt instrument face amount	$ 787,900,000	$ 701,000,000	$ 325,000,000	$ 325,000,000	$ 325,000,000
Maturity date			May 15, 2019
Length of credit facility, in years						5 years	5 years
Credit facility maximum borrowing capacity							450,000,000
Interest rate of debt instrument				6.75%
Interest payable terms			semiannually, in arrears, on May 15 and November 15
Unamortized debt discounts	10,200,000	30,400,000	2,500,000
Payments of debt issuance costs	7,300,000	7,100,000	7,100,000
Proceeds from issuance of long term debt after issuance costs			315,400,000
Net proceeds from sale of notes	$ 175,000,000	$ 322,500,000	$ 322,500,000
Effective annual interest rate			6.875%