Notes and Bonds Payable (Details) (USD $)	3 Months Ended		6 Months Ended		0 Months Ended		3 Months Ended	0 Months Ended			1 Months Ended			0 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 28, 2013 TIAA 7.25% secured loan	Feb. 15, 2013 Line of Credit [Member] Unsecured Credit Facility Due 2015 [Member] extension_option	Jun. 30, 2013 Line of Credit [Member] Unsecured Credit Facility Due 2015 [Member]	Feb. 15, 2013 Line of Credit [Member] Unsecured Credit Facility Due 2015 [Member] Minimum	Feb. 15, 2013 Line of Credit [Member] Unsecured Credit Facility Due 2015 [Member] Maximum	Jun. 03, 2013 Mortgages [Member]	Feb. 28, 2013 Mortgages [Member] 6.55% Mortage Note Payable [Member]	Mar. 26, 2013 Senior Notes [Member] 3.75% Senior Notes Due 2023 [Member]	Mar. 26, 2013 Senior Notes [Member] 3.75% Senior Notes Due 2023 [Member] Other Assets	Apr. 18, 2013 Senior Notes [Member] 5.125% Senior Notes Due 2014 [Member]	Jun. 30, 2013 Senior Notes [Member] Senior Notes Due 2017 [Member]
Debt Instrument [Line Items]
Number of extension options						2
Period of extension option						6 months
Extension fees as percentage of aggregate commitments						0.075%
Percentage of interest in addition to LIBOR						1.40%		0.95%	1.75%
Facility fee on the aggregate amount of commitments						0.30%		0.15%	0.35%
Deferred finance costs						$ 2,700,000								$ 200,000
Write off of unamortized deferred financing costs					500,000		300,000
Amount outstanding under credit facility							230,000,000
Remaining borrowing capacity							470,000,000
Repayments of Notes Payable										1,100,000	14,900,000
Debt face amount												250,000,000.0
Debt interest rate					7.25%						6.55%	3.75%		5.125%
Debt Instrument, Unamortized Discount												2,100,000
Debt yielded interest rate												3.85%
Unamortized Debt Issuance Expense													2,100,000
Extinguishment of debt amount					94,300,000									277,300,000
Extinguishment of debt outstanding principal amount					77,000,000									264,700,000
Extinguishment of debt, accrued interest amount					500,000									700,000
Prepayment penalties					16,800,000					300,000
Make whole amount on extinguishment of debt														11,900,000
Write off of unamortized discount														200,000
Loss on extinguishment of debt	(29,638,000)	0	(29,638,000)	0	17,400,000									12,300,000
Authorized Repurchase Of Debt Amount															$ 50,000,000