Unsecured Debt and Lines of Credit Debt and Lines Of Credit Summary (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Unsecured debt	$ 2,617,482,000	$ 1,410,023,000
Debt Instrument, Face Amount	1,900,000,000	600,000,000
Lines of credit	246,391,000	219,421,000
Unsecured Debt And Line Of Credit	2,863,873,000	1,629,444,000
Line Of Credit Facility Maturity Date 1
Fixed Rate Bond One [Member]
Debt Instrument [Line Items]
Unsecured debt	465,000,000	465,000,000
Debt Instruments Maturity Date	4 years 2 months 24 days
Loans Payable [Member]
Debt Instrument [Line Items]
Unsecured debt		350,000,000
Debt Instrument, Face Amount	225,000,000	350,000,000
Long-term Debt, Weighted Average Interest Rate	2.40%	2.50%
Debt Instruments Maturity Date	1 year 10 months 24 days
Fixed Rate Bond Two [Member]
Debt Instrument [Line Items]
Unsecured debt	$ 1,927,482,000	$ 595,023,000
Debt Instruments Maturity Date	7 years 1 month 24 days
Unsecured Bonds [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	3.60%	4.00%
Unsecured Line of Credit [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	1.80%	2.20%