Subsequent events Narrative (Details) (USD $)	3 Months Ended		6 Months Ended		0 Months Ended		6 Months Ended	0 Months Ended				0 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jul. 18, 2014 Subsequent Event [Member]	Jul. 15, 2014 Subsequent Event [Member] Parcel	Jun. 30, 2014 Senior Notes [Member]	Jul. 18, 2014 Unsecured Senior Notes Due in 2020 and 2029 [Member] Senior Notes [Member] Subsequent Event [Member]	Jul. 18, 2014 Unsecured Senior Notes Due in 2020 and 2029 [Member] Senior Notes [Member] Subsequent Event [Member]	Jul. 18, 2014 Unsecured Senior Notes Due in January 2020 [Member] Senior Notes [Member] Subsequent Event [Member]	Jul. 18, 2014 Unsecured Senior Notes Due in July 2029 [Member] Senior Notes [Member] Subsequent Event [Member]	Jul. 18, 2014 2016 Unsecured Senior Bank Term Loan Subsequent Event [Member]	Jul. 18, 2014 Unsecured Credit Facility [Member] Subsequent Event [Member]
Subsequent Event [Line Items]
Debt Instrument, Face Amount									$ 700,000,000	$ 400,000,000	$ 300,000,000
Stated interest rate (as a percent)									3.50%	2.75%	4.50%
Weighted Average Remaining Terms (in years)			5 years 1 month				8 years 3 months	9 years 7 months
Proceeds from Unsecured Notes Payable								694,000,000
Repayments of Unsecured Debt			0	150,000,000								125,000,000	569,000,000
Gains (Losses) on Extinguishment of Debt	0	560,000	0	560,000	500,000
Number of Real Estate Properties Sold						2
Proceeds from Sale of Real Estate			19,000,000			7,900,000
Gain (Loss) on Sale of Properties, Net of Applicable Income Taxes	$ 797,000	$ 772,000	$ 797,000	$ 772,000		$ 207,000