Secured and unsecured senior debt Summary of secured and unsecured debt (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
Fixed Rate/Hedged Variable Rate	3,103,415,000			3,103,415,000
Unhedged Variable rate	397,700,000			397,700,000
Total Consolidated	3,501,115,000			3,501,115,000
Percentage of Total	100.00%			100.00%
Total consolidated debt, Effective Interest	3.39%	[1]		3.39%	[1]
Weighted Average Remaining Terms (in years)				5 years 11 months
Percentage of fixed rate/hedged total debt	89.00%			89.00%
Percentage of unhedged floating rate total debt	11.00%			11.00%
Gains (Losses) on Extinguishment of Debt	(525,000)		(1,432,000)	(525,000)		(1,992,000)
Secured notes payable
Debt Instrument [Line Items]
Fixed Rate/Hedged Variable Rate	406,125,000			406,125,000
Unhedged Variable rate	230,700,000			230,700,000
Total Consolidated	636,825,000			636,825,000
Percentage of Total	18.20%			18.20%
Weighted Average Interest Rate at End of Period	4.67%	[1]		4.67%	[1]
Weighted Average Remaining Terms (in years)				2 years 11 months
Stated interest rate (as a percent)	4.71%			4.71%
Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	700,000,000			700,000,000
Fixed Rate/Hedged Variable Rate	1,747,290,000			1,747,290,000
Unhedged Variable rate	0			0
Total Consolidated	1,747,290,000			1,747,290,000
Percentage of Total	49.90%			49.90%
Weighted Average Interest Rate at End of Period	3.98%			3.98%
Weighted Average Remaining Terms (in years)				8 years 7 months
Stated interest rate (as a percent)	3.50%			3.50%
Subsidiary or Equity Method Investee, Cumulative Percentage Ownership after All Transactions				100.00%
$1.5 billion unsecured senior line of credit
Debt Instrument [Line Items]
Outstanding Balance	142,000,000			142,000,000
Fixed Rate/Hedged Variable Rate	0			0
Unhedged Variable rate	142,000,000			142,000,000
Total Consolidated	142,000,000			142,000,000
Percentage of Total	4.10%			4.10%
Weighted Average Interest Rate at End of Period	1.25%	[1]		1.25%	[1]
Weighted Average Remaining Terms (in years)				4 years 3 months
2016 Unsecured Senior Bank Term Loan
Debt Instrument [Line Items]
Outstanding Balance	375,000,000			375,000,000
Fixed Rate/Hedged Variable Rate	350,000,000			350,000,000
Unhedged Variable rate	25,000,000			25,000,000
Total Consolidated	375,000,000			375,000,000
Percentage of Total	10.70%			10.70%
Weighted Average Interest Rate at End of Period	1.42%	[1]		1.42%	[1]
Weighted Average Remaining Terms (in years)				1 year 9 months
2019 Unsecured Senior Bank Term Loan
Debt Instrument [Line Items]
Outstanding Balance	600,000,000			600,000,000
Fixed Rate/Hedged Variable Rate	600,000,000			600,000,000
Unhedged Variable rate	0			0
Total Consolidated	$ 600,000,000			$ 600,000,000
Percentage of Total	17.10%			17.10%
Weighted Average Interest Rate at End of Period	2.05%	[1]		2.05%	[1]
Weighted Average Remaining Terms (in years)				4 years 3 months

[1] Represents the weighted average interest rate as of the end of the period plus the impact of debt premiums/discounts and our interest rate swap agreements. The weighted average interest rate excludes bank fees and amortization of loan fees.