Secured and unsecured senior debt Unsecured senior notes payable (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Debt Instrument [Line Items]
Repayments of Unsecured Debt			$ 125,000,000	$ 250,000,000
Gains (Losses) on Extinguishment of Debt	(525,000)	(1,432,000)	(525,000)	(1,992,000)
Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	700,000,000		700,000,000
Stated interest rate (as a percent)	3.50%		3.50%
Subsidiary or Equity Method Investee, Cumulative Percentage Ownership after All Transactions			100.00%
Senior Notes [Member] | 2016 Unsecured Senior Bank Term Loan
Debt Instrument [Line Items]
Repayments of Unsecured Debt			125,000,000
Senior Notes [Member] | $1.5 billion unsecured senior line of credit
Debt Instrument [Line Items]
Repayments of Unsecured Debt			569,000,000
Unsecured Senior Notes Due in January 2020 [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	400,000,000		400,000,000
Stated interest rate (as a percent)	2.80%		2.80%
Debt Instrument, Issuance Price as Percentage of Principal Amount			99.793%
Debt Instrument, Yield Rate			2.791%
Unsecured Senior Notes Due in July 2029 [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 300,000,000		$ 300,000,000
Stated interest rate (as a percent)	4.50%		4.50%
Debt Instrument, Issuance Price as Percentage of Principal Amount			99.912%
Debt Instrument, Yield Rate			4.508%