Interest rate swap agreements Outstanding interest rate swap (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013
Interest rate hedge agreements
Fair Values	$ (3,321)
Notional Amount in Effect as of 2013	950,000
Notional Amount in Effect as of 2014	750,000
Notional Amount in Effect as of 2015	500,000
Notional Amount in Effect as of 2016	500,000
Unsecured Bank Term Loan 2016 | Minimum
Interest rate hedge agreements
Applicable margin (as a percent)	1.20%
Unsecured Senior Line of Credit and Bank Term Loans | Minimum
Interest rate hedge agreements
Applicable margin (as a percent)	1.10%
Transaction Date December 2006, Interest Pay Rate 4.990%
Interest rate hedge agreements
Interest Pay Rate (as a percent)	4.99%	[1]
Fair Values	(596)
Notional Amount in Effect as of 2013	50,000
Notional Amount in Effect as of 2014	0
Notional Amount in Effect as of 2015	0
Notional Amount in Effect as of 2016	0
Transaction Date November 2009, Interest Pay Rate 5.020%
Interest rate hedge agreements
Interest Pay Rate (as a percent)	5.02%	[1]
Fair Values	(1,800)
Notional Amount in Effect as of 2013	150,000
Notional Amount in Effect as of 2014	0
Notional Amount in Effect as of 2015	0
Notional Amount in Effect as of 2016	0
Transaction Date December 2013, Interest Pay Rate .980%
Interest rate hedge agreements
Interest Pay Rate (as a percent)	0.98%	[1]
Fair Values	(3,795)
Notional Amount in Effect as of 2013	500,000
Notional Amount in Effect as of 2014	0
Notional Amount in Effect as of 2015	0
Notional Amount in Effect as of 2016	0
Interest Rate Hedge .23 Percent Transaction Date March 2015
Interest rate hedge agreements
Interest Pay Rate (as a percent)	0.23%	[1]
Fair Values	66
Notional Amount in Effect as of 2013	250,000
Notional Amount in Effect as of 2014	250,000
Notional Amount in Effect as of 2015	0
Notional Amount in Effect as of 2016	0
Transaction date December 2014, interest pay rate 0.527%
Interest rate hedge agreements
Interest Pay Rate (as a percent)	0.53%	[1]
Fair Values	658
Notional Amount in Effect as of 2013	0
Notional Amount in Effect as of 2014	500,000
Notional Amount in Effect as of 2015	500,000
Notional Amount in Effect as of 2016	0
Transaction date March 2016, interest pay rate 1.406%
Interest rate hedge agreements
Interest Pay Rate (as a percent)	1.40%	[1]
Fair Values	2,146
Notional Amount in Effect as of 2013	0
Notional Amount in Effect as of 2014	0
Notional Amount in Effect as of 2015	0
Notional Amount in Effect as of 2016	$ 500,000

[1] In addition to the interest pay rate, borrowings outstanding as of December 31, 2013, 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%