Unsecured Senior Notes Unsecured Senior Notes (Narrative) (Details) (USD $)	12 Months Ended			0 Months Ended			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jun. 11, 2012 Eleven Year Unsecured Senior Notes Three Point Nine Five Four Percent		Aug. 24, 2012 Ten Year Unsecured Senior Notes Six Point Two Five Percent [Member]	Dec. 31, 2012 Senior Notes [Member]	Dec. 31, 2012 Senior Notes [Member] Eleven Year Unsecured Senior Notes Three Point Nine Five Four Percent
Debt Instrument [Line Items]
Debt Instrument, Face Amount				$ 1,000,000,000
Debt Instrument, Redemption Date						Aug. 24, 2012
Debt Instrument, Repurchased Face Amount						225,000,000
Coupon/Stated Rate				3.85%		6.25%		3.85%
Senior Notes Pricing				99.779%
Effective Rate				3.954%	[1]			3.954%	[1]
Maturity date				Feb. 01, 2023				Feb. 01, 2023	[2]
Proceeds from unsecured senior notes	997,790,000	848,019,000	1,542,947,000	989,400,000
Repayments of Unsecured Debt	224,261,000	0	700,000,000			231,600,000
Accrued and Unpaid Interest for Repurchased Debt						1,500,000
Redemption Price						102.25%
Losses from early extinguishments of debt	4,453,000	1,494,000	89,883,000			5,200,000
Payment of Debt Redemption Premium						$ 5,100,000
Maximum Leverage Ratio							60.00%
Maximum Secured Debt Leverage Ratio	55.00%						50.00%
Minimum Interest Coverage Ratio							1.50
Minimum Unencumbered Asset Value Percentage Of Unsecured Debt							150.00%

[1] Yield on issuance date including the effects of discounts on the notes.

[2] No principal amounts are due prior to maturity.