Unsecured Debt and Lines of Credit (Details) (USD $)				6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended
	Jun. 30, 2014	Apr. 30, 2014	Dec. 31, 2013	Jun. 30, 2014 Bonds Private Placement - Fixed Rate [Member]	Dec. 31, 2013 Bonds Private Placement - Fixed Rate [Member]	Jun. 30, 2014 Term Loan - Variable Rate [Member]	Dec. 31, 2013 Term Loan - Variable Rate [Member]	Jun. 30, 2014 Bonds Public Offering - Fixed Rate [Member]	Dec. 31, 2013 Bonds Public Offering - Fixed Rate [Member]	Jun. 30, 2014 Line of Credit [Member]	Dec. 31, 2013 Line of Credit [Member]	Apr. 30, 2014 5.5% senior notes due 2017 [Member]	Apr. 30, 2014 5.2% seinior notes due 2021 [Member]	Apr. 30, 2014 3.375% senior notes due 2023 [Member]
Debt Instrument [Line Items]
Aggregate principal amount of senior notes assumed		$ 900,000,000
Principal balance on debt		711,300,000
Premium on debt assumed		124,500,000
Unsecured debts and line of credit [Abstract]
Unsecured debt	2,748,494,000		1,410,023,000	465,000,000	465,000,000	350,000,000	350,000,000	1,933,494,000	595,023,000
Lines of credit	66,975,000		219,421,000
Total unsecured debt and lines of credit	2,815,469,000		1,629,444,000
Weighted Average Maturity				4 years 8 months 12 days		2 years 8 months 12 days		7 years 6 months
Weighted Average Maturity, Line of credit										4 years 8 months 12 days
Weighted average interest rate (in hundredths)		5.60%				2.40%	2.50%	3.70%	4.00%	1.70%	2.20%	5.50%	5.20%	3.375%
Aggregate schedule principal payments [Abstract]
2014	0
2015	66,721,000
2016	369,225,000
2017	669,962,000
2018	313,541,000
Thereafter	3,522,146,000
Total	$ 4,941,595,000