Unsecured Debt and Lines of Credit Line of Credit (Details) (USD $)	12 Months Ended
	Dec. 31, 2014 line_of_credit	Dec. 31, 2013
Line of Credit Facility [Line Items]
Line Of Credit Facility Number of outstanding lines	2
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,030,000,000
Lines of credit	246,391,000	219,421,000
Unsecured Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of credit facility Number of one year extensions available	1
Debt Instrument Basis spread on variable rate, initial	0.95%
Line Of Credit Facility Maximum Borrowing Capacity Accordion Option	1,000,000,000
Variable rate basis	LIBOR
Lines of credit	229,800,000	199,000,000
Line Of Credit Facility, Period For Extension Option	18 months
Line of Credit Working Capital [Member]
Line of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity Accordion Option	25,000,000.0
Variable rate basis	LIBOR
Lines of credit	$ 16,600,000	$ 20,421,000
Line Of Credit Facility, Period For Extension Option	1 year