Interest rate swap agreements Outstanding interest rate swap (Details) $ in Thousands		9 Months Ended
		Sep. 30, 2015 USD ($) contract	Dec. 31, 2014 USD ($)
Interest rate hedge agreements
Effective Portion of Interest Rate Cash Flow Hedge Derivatives		$ 1,050,000
Notional Amount in Effect as of 12/31/15		1,050,000
Notional Amount in Effect as of 12/31/16		1,100,000
Effective Portion of Interest Rate Cash Flow Hedge in Year Three		650,000
Interest Rate Derivative Liabilities, at Fair Value		$ 8,679	$ 909
Unsecured Bank Term Loan 2016 | Minimum
Interest rate hedge agreements
Applicable margin (as a percent)		1.20%
Unsecured Credit Facility [Member] | Minimum
Interest rate hedge agreements
Applicable margin (as a percent)		1.10%
Interest Rate Hedge 0.527 Percent Transaction Date December 2014 Contract Three [Member]
Interest rate hedge agreements
Number of Contracts | contract		3
Interest Pay Rate (as a percent)	[1]	0.53%
Fair Values		$ (627)
Effective Portion of Interest Rate Cash Flow Hedge Derivatives		500,000
Notional Amount in Effect as of 12/31/15		500,000
Notional Amount in Effect as of 12/31/16		0
Effective Portion of Interest Rate Cash Flow Hedge in Year Three		$ 0
Interest Rate Hedge .42 Percent Transaction Date March 2015 [Member]
Interest rate hedge agreements
Number of Contracts | contract		7
Interest Pay Rate (as a percent)	[1]	0.42%
Fair Values		$ (331)
Effective Portion of Interest Rate Cash Flow Hedge Derivatives		450,000
Notional Amount in Effect as of 12/31/15		450,000
Notional Amount in Effect as of 12/31/16		0
Effective Portion of Interest Rate Cash Flow Hedge in Year Three		$ 0
Interest Rate Hedge .57 Percent Transaction Date September 2015 [Member]
Interest rate hedge agreements
Number of Contracts | contract		2
Interest Pay Rate (as a percent)	[1]	0.57%
Fair Values		$ (85)
Effective Portion of Interest Rate Cash Flow Hedge Derivatives		100,000
Notional Amount in Effect as of 12/31/15		100,000
Notional Amount in Effect as of 12/31/16		100,000
Effective Portion of Interest Rate Cash Flow Hedge in Year Three		$ 0
Interest Rate Hedge 1.35 Percent Transaction Date March 2016 [Member]
Interest rate hedge agreements
Number of Contracts | contract		11
Interest Pay Rate (as a percent)	[1]	1.15%
Fair Values		$ (5,149)
Effective Portion of Interest Rate Cash Flow Hedge Derivatives		0
Notional Amount in Effect as of 12/31/15		0
Notional Amount in Effect as of 12/31/16		1,000,000
Effective Portion of Interest Rate Cash Flow Hedge in Year Three		$ 0
Interest Rate Hedges Maturing March 2018 [Member]
Interest rate hedge agreements
Number of Contracts | contract		11
Interest Pay Rate (as a percent)	[1]	1.51%
Fair Values		$ (2,487)
Effective Portion of Interest Rate Cash Flow Hedge Derivatives		0
Notional Amount in Effect as of 12/31/15		0
Notional Amount in Effect as of 12/31/16		0
Effective Portion of Interest Rate Cash Flow Hedge in Year Three		$ 650,000

[1] In addition to the interest pay rate for each swap agreement, interest is payable at an applicable margin for borrowings outstanding as of September 30, 2015. Borrowings under our 2019 unsecured senior bank term loan (“2019 Unsecured Senior Bank Term Loan”) include an applicable margin of 1.20%, and borrowings outstanding under our unsecured senior line of credit and 2021 Unsecured Senior Bank Term Loan include an applicable margin of 1.10%.