Noncontrolling Interests (Redeemable Preferred Units) (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified		0 Months Ended	3 Months Ended	9 Months Ended										3 Months Ended		9 Months Ended					0 Months Ended	9 Months Ended		9 Months Ended
	Sep. 30, 2013 Noncontrolling Interests [Member]	Oct. 21, 2013 Series Two Preferred Units [Member]	Sep. 30, 2013 Series Two Preferred Units [Member]	Sep. 30, 2012 Series Two Preferred Units [Member]	Sep. 30, 2013 Series Two Preferred Units [Member] Noncontrolling Interests [Member]	Aug. 15, 2013 Series Two Preferred Units [Member] Noncontrolling Interests [Member]	May 15, 2013 Series Two Preferred Units [Member] Noncontrolling Interests [Member]	May 14, 2013 Series Two Preferred Units [Member] Noncontrolling Interests [Member]	Feb. 15, 2013 Series Two Preferred Units [Member] Noncontrolling Interests [Member]	Dec. 31, 2012 Series Two Preferred Units [Member] Noncontrolling Interests [Member]	Dec. 31, 2011 Series Two Preferred Units [Member] Noncontrolling Interests [Member]	Dec. 31, 2010 Series Two Preferred Units [Member] Noncontrolling Interests [Member]	Dec. 31, 2009 Series Two Preferred Units [Member] Noncontrolling Interests [Member]	Sep. 30, 2013 series four preferred units [Member]	Aug. 07, 2013 series four preferred units [Member]	Sep. 30, 2013 series four preferred units [Member] Noncontrolling Interests [Member]	Aug. 15, 2013 series four preferred units [Member] Noncontrolling Interests [Member]	May 29, 2013 series four preferred units [Member] Noncontrolling Interests [Member] D	May 15, 2013 series four preferred units [Member] Noncontrolling Interests [Member]	Feb. 15, 2013 series four preferred units [Member] Noncontrolling Interests [Member]	Oct. 21, 2013 OP Units [Member]	Sep. 30, 2012 OP Units [Member]	Sep. 30, 2013 OP Units [Member]	Sep. 30, 2013 Redemption Year Two Thousand Fourteen [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Conversion of Stock, Shares Converted		329,881		117,407																	432,914	153,605
Maximum Redeemable On Redemption Dates Preferred Units																								1,007,662
Series Two Preferred Units (in shares)	995,997				995,997
Liquidation preference percentage					6.00%
Operating Units liquidation preference (in dollars per share)					$ 38.10
OP Units for each Preferred Unit (in shares)			1.312336		1.312336
Series Two Preferred Units redemption price per unit (in dollars per share)																								$ 50.00
Dates Series Two Preferred holders can require Operating Partnership to redeem their units for cash																								May 12, 2014
Distributions to Series Two Preferred Units (in dollars per share)						$ 0.85302	$ 0.85302		$ 0.85302								$ 0.25		$ 0.25	$ 0.25
Maximum portion that may be required for redemption of total																								0.1666666667
Value of Series Two Preferred Units assuming conversion																							$ 139.7
Closing price of common stock (in dollars per share)																							$ 106.90
Series Four Preferred Units (in shares)	360,126															360,126
Series Four Preferred Units Annual Dividend Payable Rate														2.00%		2.00%
Preferred Units Liquidation Preference			$ 50.00		$ 50.00									$ 50.00	$ 50.00	$ 50.00
First opportunity to redeem Series Four Preferred Units (in days)																		30
Series Four Preferred Units Redeemed															861,400
Number Of Holders Exercises								0		0	0	0	0