Interest rate swap agreements Outstanding interest rate swap (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014
Interest rate hedge agreements
Interest Rate Derivative Liabilities, at Fair Value	$ 2,600
Fair Values	(2,622)
Effective Portion of Interest Rate Cash Flow Hedge Derivatives	950,000
Notional Amount in Effect as of 12/31/14	950,000
Notional Amount in Effect as of 12/31/15	500,000
Notional Amount in Effect as of 12/31/16	500,000
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%
Transaction Date December 2013, Interest Pay Rate .980%
Interest rate hedge agreements
Number of Contracts	2
Interest Pay Rate (as a percent)	0.98%	[1]
Fair Values	(2,114)
Effective Portion of Interest Rate Cash Flow Hedge Derivatives	500,000
Notional Amount in Effect as of 12/31/14	0
Notional Amount in Effect as of 12/31/15	0
Notional Amount in Effect as of 12/31/16	0
Interest Rate Hedge .23 Percent Transaction Date December 2013
Interest rate hedge agreements
Number of Contracts	2
Interest Pay Rate (as a percent)	0.23%	[1]
Fair Values	(144)
Effective Portion of Interest Rate Cash Flow Hedge Derivatives	250,000
Notional Amount in Effect as of 12/31/14	250,000
Notional Amount in Effect as of 12/31/15	0
Notional Amount in Effect as of 12/31/16	0
Interest Rate Hedge .21 Percent Transaction Date March 2014 [Member]
Interest rate hedge agreements
Number of Contracts	4
Interest Pay Rate (as a percent)	0.21%	[1]
Fair Values	(75)
Effective Portion of Interest Rate Cash Flow Hedge Derivatives	200,000
Notional Amount in Effect as of 12/31/14	200,000
Notional Amount in Effect as of 12/31/15	0
Notional Amount in Effect as of 12/31/16	0
Transaction date December 2014, interest pay rate 0.527%
Interest rate hedge agreements
Number of Contracts	3
Interest Pay Rate (as a percent)	0.53%	[1]
Fair Values	(335)
Effective Portion of Interest Rate Cash Flow Hedge Derivatives	0
Notional Amount in Effect as of 12/31/14	500,000
Notional Amount in Effect as of 12/31/15	500,000
Notional Amount in Effect as of 12/31/16	0
Transaction date March 2016, interest pay rate 1.406%
Interest rate hedge agreements
Number of Contracts	3
Interest Pay Rate (as a percent)	1.40%	[1]
Fair Values	46
Effective Portion of Interest Rate Cash Flow Hedge Derivatives	0
Notional Amount in Effect as of 12/31/14	0
Notional Amount in Effect as of 12/31/15	0
Notional Amount in Effect as of 12/31/16	$ 500,000

[1] In addition to the interest pay rate, borrowings outstanding as of June 30, 2014, under our unsecured senior bank term loans include an applicable margin of 1.20% and borrowings outstanding under our unsecured senior line of credit include an applicable margin of 1.10%.