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		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		1 Months Ended		3 Months Ended
	Dec. 31, 2013 LineofCredit	Dec. 31, 2012	Dec. 31, 2013 Unsecured Line of Credit [Member] Extension	Dec. 31, 2012 Unsecured Line of Credit [Member]	Dec. 31, 2013 Line of Credit Working Capital [Member]	Dec. 31, 2012 Line of Credit Working Capital [Member]	Dec. 31, 2013 Bonds Private Placement - Fixed Rate [Member]	Dec. 31, 2012 Bonds Private Placement - Fixed Rate [Member]	Dec. 31, 2013 Term Loan - Variable Rate [Member]	Dec. 31, 2012 Term Loan - Variable Rate [Member]	Dec. 31, 2011 Term Loan - Variable Rate [Member]	Dec. 31, 2013 Bonds Public Offering - Fixed Rate [Member]	Dec. 31, 2012 Bonds Public Offering - Fixed Rate [Member]	Dec. 31, 2013 Line of Credit [Member]	Dec. 31, 2012 Line of Credit [Member]	Dec. 31, 2013 Unsecured Bonds [Member]	Dec. 31, 2012 Unsecured Bonds [Member]	Dec. 31, 2013 Unsecured Bonds 4.36% [Member]	Dec. 31, 2012 Unsecured Bonds 4.36% [Member]	Dec. 31, 2013 Unsecured Bonds 4.5% [Member]	Dec. 31, 2012 Unsecured Bonds 4.5% [Member]	Dec. 31, 2013 Unsecured Bonds 4.92 % [Member]	Dec. 31, 2012 Unsecured Bonds 4.92 % [Member]	Dec. 31, 2013 Unsecured Bonds 4.27% [Member]	Dec. 31, 2012 Unsecured Bonds 4.27% [Member]	Dec. 31, 2013 Unsecured Bonds 4.30% [Member]	Dec. 31, 2012 Unsecured Bonds 4.30% [Member]	Dec. 31, 2013 Unsecured Bonds 4.37% [Member]	Dec. 31, 2012 Unsecured Bonds 4.37% [Member]	Apr. 30, 2013 Unsecured Bonds 3.25% [Member]	Dec. 31, 2013 Unsecured Bonds 3.25% [Member]	Sep. 30, 2013 Unsecured Bonds 3.625% [Member]	Dec. 31, 2013 Unsecured Bonds 3.625% [Member]	Dec. 31, 2012 Unsecured Bonds 3.625% [Member]
Unsecured debts and line of credit [Abstract]
Unsecured debt	$ 1,410,023,000	$ 1,112,084,000					$ 465,000,000	$ 465,000,000	$ 350,000,000	$ 350,000,000		$ 595,023,000	$ 297,084,000
Lines of credit	219,421,000	141,000,000
Total unsecured debt	1,629,444,000	1,253,084,000
Weighted Average Maturity							5 years 2 months 12 days		3 years 2 months 12 days			9 years
Weighted Average Maturity, Line of credit	4 years 3 months 18 days
Weighted average interest rate (in hundredths)									2.50%	2.70%				2.20%	2.30%	4.00%	4.20%
Senior Notes [Abstract]
Maturity date																		Mar. 31, 2016		Sep. 30, 2017		Dec. 31, 2019		Apr. 30, 2021		Jun. 30, 2021		Aug. 31, 2021		May 01, 2023		Aug. 15, 2022
Senior Notes	465,000,000	465,000,000																150,000,000	150,000,000	40,000,000	40,000,000	75,000,000	75,000,000	100,000,000	100,000,000	50,000,000	50,000,000	50,000,000	50,000,000
Coupon Rate (in hundredths)																			4.36%		4.50%		4.92%		4.27%		4.30%		4.37%	3.25%		3.625%
Proceeds from issuance of unsecured bonds																														300,000,000		300,000,000
Carrying value of bonds, net of discount																															297,700,000		297,300,000	297,100,000
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	350,000,000	200,000,000
Debt offering price (in hundredths)																															99.152%		98.99%
Line of Credit Facility [Line Items]
Number of outstanding lines of credit	2
Line of credit maximum borrowing capacity	625,000,000
Line of credit with an accordion option			600,000,000		25,000,000
Unsecured line of credit balance			$ 199,000,000	$ 141,000,000	$ 20,400,000	$ 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