Unsecured Debt and Lines of Credit (Details) (USD $)	12 Months Ended		12 Months Ended				12 Months Ended								12 Months Ended		12 Months Ended		12 Months Ended		3 Months Ended	12 Months Ended		3 Months Ended	12 Months Ended		3 Months Ended	12 Months Ended		3 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Unsecured Line of Credit [Member]	Dec. 31, 2012 Line of Credit Working Capital [Member]	Dec. 31, 2012 Bonds Private Placement - Fixed Rate [Member]	Dec. 31, 2011 Bonds Private Placement - Fixed Rate [Member]	Dec. 31, 2012 Term Loan - Variable Rate [Member]	Dec. 31, 2011 Term Loan - Variable Rate [Member]	Dec. 31, 2012 Bonds Public Offering - Fixed Rate [Member]	Dec. 31, 2011 Bonds Public Offering - Fixed Rate [Member]	Dec. 31, 2012 Line of Credit [Member]	Dec. 31, 2011 Line of Credit [Member]	Dec. 31, 2012 Unsecured Bonds [Member]	Dec. 31, 2011 Unsecured Bonds [Member]	Dec. 31, 2012 Unsecured Bonds 4.36% [Member]	Dec. 31, 2011 Unsecured Bonds 4.36% [Member]	Dec. 31, 2012 Unsecured Bonds 4.5% [Member]	Dec. 31, 2011 Unsecured Bonds 4.5% [Member]	Dec. 31, 2012 Unsecured Bonds 4.92 % [Member]	Dec. 31, 2011 Unsecured Bonds 4.92 % [Member]	Jun. 30, 2012 Unsecured Bonds 4.27% [Member]	Dec. 31, 2012 Unsecured Bonds 4.27% [Member]	Dec. 31, 2011 Unsecured Bonds 4.27% [Member]	Jun. 30, 2012 Unsecured Bonds 4.30% [Member]	Dec. 31, 2012 Unsecured Bonds 4.30% [Member]	Dec. 31, 2011 Unsecured Bonds 4.30% [Member]	Sep. 30, 2012 Unsecured Bonds 4.37% [Member]	Dec. 31, 2012 Unsecured Bonds 4.37% [Member]	Dec. 31, 2011 Unsecured Bonds 4.37% [Member]	Sep. 30, 2012 Unsecured Bonds 3.625% [Member]	Dec. 31, 2012 Unsecured Bonds 3.625% [Member]
Unsecured debts and line of credit [Abstract]
Unsecured debt	$ 1,112,084,000	$ 465,000,000			$ 465,000,000	$ 265,000,000	$ 350,000,000	$ 200,000,000	$ 297,084,000	$ 0
Lines of credit	141,000,000	150,000,000
Total unsecured debt	1,253,084,000	615,000,000
Weighted Average Maturity					6 years 2 months 12 days		4 years 2 months 12 days		9 years 7 months 6 days
Weighted Average Maturity, Line of credit	3 years
Weighted average interest rate (in hundredths)							2.70%	2.60%			2.30%	2.50%	4.20%	4.50%
Senior Notes [Abstract]
Maturity date															Mar. 31, 2016		Sep. 30, 2017		Dec. 31, 2019			Apr. 30, 2021			Jun. 30, 2021			Aug. 31, 2021		Aug. 15, 2022
Senior Notes	465,000,000	265,000,000													150,000,000	150,000,000	40,000,000	40,000,000	75,000,000	75,000,000		100,000,000	0		50,000,000	0		50,000,000	0
Coupon Rae (in hundredths)																4.36%		4.50%		4.92%			4.27%			4.30%			4.37%	3.625%
Proceeds from issuance of unsecured bonds																					100,000,000			50,000,000			50,000,000			300,000,000
Debt instrument, basis spread on variable rate (in hundredths)	2.50%						1.20%
Term of loan							5 years
Notional amount of interest rate swap							300,000,000
Debt instrument face amount							350,000,000	200,000,000
Debt offering price (in hundredths)																															98.99%
Line of Credit Facility [Line Items]
Number of outstanding lines of credit	2
Line of credit maximum borrowing capacity	525,000,000
Line of credit with an accordion option			500,000,000	25,000,000
Accordion option to increase maximum borrowing capacity			600,000,000
Unsecured line of credit balance			$ 141,000,000	$ 0
Debt instrument, description of variable rate basis			LIBOR	LIBOR
Basis spread on variable rate, initial (in hundredths)			1.075%	1.075%
Number of one year extensions available			2
Line of credit facility expiration date			Dec. 31, 2015	Jan. 31, 2014