Notes and Bonds Payable (Details) (USD $)	0 Months Ended		3 Months Ended	0 Months Ended		1 Months Ended
	Apr. 18, 2013 Senior Notes [Member] 5.125% Senior Notes Due 2014 [Member] Subsequent Event [Member]	Feb. 15, 2013 Line of Credit [Member] Unsecured Credit Facility Due 2015 [Member] extension_option	Mar. 31, 2013 Line of Credit [Member] Unsecured Credit Facility Due 2015 [Member]	Feb. 15, 2013 Line of Credit [Member] Unsecured Credit Facility Due 2015 [Member] Maximum [Member]	Feb. 15, 2013 Line of Credit [Member] Unsecured Credit Facility Due 2015 [Member] Minimum [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 [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%		1.75%	0.95%
Facility fee on the aggregate amount of commitments		0.30%		0.35%	0.15%
Deferred finance costs		$ 2,700,000
Write off of unamortized deferred financing costs	200,000		300,000
Amount outstanding under unsecured credit facility			0
Remaining borrowing capacity			700,000,000
Repayments of Notes Payable						14,900,000
Debt face amount							250,000,000.0
Debt interest rate						6.55%	3.75%
Debt Instrument, Unamortized Discount							2,100,000
Debt yielded interest rate							3.85%
Unamortized Debt Issuance Expense								2,100,000
Extinguishment of debt amount	277,300,000
Extinguishment of debt outstanding principal amount	264,700,000
Extinguishment of debt, accrued interest amount	700,000
Make whole amount on extinguishment of debt	11,900,000
Write off of unamortized discount	200,000
Loss on extinguishment of debt	$ (12,300,000)