Unsecured Senior Notes Unsecured Senior Notes (Narrative) (Details) (USD $)	6 Months Ended			0 Months Ended	6 Months Ended		0 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Senior Notes [Member]	Apr. 11, 2013 3.279% Unsecured Senior Notes [Member]	Jun. 30, 2013 3.279% Unsecured Senior Notes [Member]		Jun. 27, 2013 3.916%t Unsecured Senior Notes [Member]	Jun. 30, 2013 3.916%t Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Maximum Leverage Ratio			60.00%
Maximum Secured Debt Leverage Ratio			50.00%
Minimum Interest Coverage Ratio			1.50
Minimum Unencumbered Asset Value Percentage Of Unsecured Debt			150.00%
Proceeds from Issuance of Unsecured Debt	$ 1,194,753,000	$ 997,790,000		$ 492,500,000			$ 691,900,000
Debt Instrument, Face Amount				$ 500,000,000			$ 700,000,000
Coupon/Stated Rate				3.125%	3.125%		3.80%	3.80%
Senior Notes Pricing				99.379%			99.694%
Effective Percentage				3.279%	3.279%	[1]	3.916%	3.916%	[1]
Debt Instrument, Maturity Date				Sep. 01, 2023	Sep. 01, 2023	[2]	Feb. 01, 2024	Feb. 01, 2024	[2]

[1] Yield on issuance date including the effects of discounts on the notes and the amortization of financing costs.

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