Senior Unsecured Notes and Secured Debt (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended				6 Months Ended						12 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Jul. 17, 2012	Dec. 31, 2011	Jun. 30, 2012 Senior Unsecured Notes [Member]		Jun. 30, 2012 Secured Debt [Member]		Jun. 30, 2012 Excluding Fair Value Adjustments [Member]	Jun. 30, 2011 Excluding Fair Value Adjustments [Member]	Dec. 31, 2011 Excluding Fair Value Adjustments [Member]	Dec. 31, 2010 Excluding Fair Value Adjustments [Member]
Principal payments due on debt obligations
2012	$ 264,549		$ 264,549				$ 244,939	[1],[2],[3]	$ 19,610	[1],[4]
2013	520,373		520,373				300,000	[1],[2]	220,373	[1],[4]
2014	201,317		201,317				0	[1],[2]	201,317	[1],[4]
2015	464,550		464,550				250,000	[1],[2]	214,550	[1],[4]
2016	1,001,528		1,001,528				700,000	[1],[2]	301,528	[1],[4]
Thereafter	4,772,468		4,772,468				3,444,403	[1],[2]	1,328,065	[1],[4]
Totals	7,224,785		7,224,785				4,939,342	[1],[2]	2,285,443	[1],[4]
Unsecured note issuances
Senior unsecured debt balance											4,939,342	4,464,930	4,464,927	3,064,930
Senior unsecured debt issued			593,319	1,381,086							600,000	1,400,000
Senior unsecured debt redeemed			125,585	0							(125,585)	0
Senior unsecured debt balance average rate											0.05021	0.05133	0.05133	0.05129
Senior unsecured debt issued average rate											0.04125	0.05143
Senior unsecured debt redeemed average rate											0.04750	0.000
Secured debt principal activity
Secured debt principal balance											2,285,443	1,869,129	2,108,373	1,133,715
Secured debt issued			139,395	58,470							139,395	58,470
Secured debt assumed											284,988	689,888
Secured debt extinguished			254,175	13,081							(229,207)	0
Secured debt principal payments											(18,106)	(12,944)
Secured debt principal balance average rate											0.0545	0.0552	0.0534	0.0559
Secured debt principal issued average rate											0.0443	0.0578
Secured debt principal assumed average rate											0.0568	0.0539
Secured debt principal extinguished average rate											0.0422	0.00
Secured debt principal payment average rate											0.0555	0.0600
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Senior unsecured notes	4,910,871		4,910,871			4,434,107
Notes, annual stated interest rates, Minimum							3.00%		1.20%
Notes, annual stated interest rates, Maximum							8.00%		8.00%
Other income less than (in excess of) cash received	(576)	0	(576)	0
Equity component of convertible debt, reacquire			2,759
Additional Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Secured debt	2,299,674		2,299,674			2,112,649
Carrying values of properties securing the debt	4,095,959		4,095,959
Senior Unsecured Convertible Notes Extinguished					$ 167,600

[1] (1) Amounts represent principal amounts due and do not include unamortized premiums/discounts or other fair value adjustments as reflected on the balance sheet.

[2] (2) Annual interest rates range from 3.0% to 8.0%.

[3] (4) We provided notice of redemption to the holders of $168,086,000 of convertible senior unsecured notes in June 2012. See Note 19 for additional information.

[4] (3) Annual interest rates range from 1.2% to 8.0%. Carrying value of the properties securing the debt totaled $4,095,959,000 at June 30, 2012.