Interest rate swap agreements Outstanding interest rate swap (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Interest rate hedge agreements
Fair Values	$ (9,319)		$ (20,700)
Notional Amount in Effect	950,000
Notional Amount in Effect at Fiscal Year End	700,000
Unsecured Senior Line of Credit and Bank Term Loans | Minimum
Interest rate hedge agreements
Applicable margin (as a percent)	1.20%
Transaction Date December 2006, Interest Pay Rate 4.990%
Interest rate hedge agreements
Interest Pay Rate (as a percent)	4.99%	[1]
Fair Values	(1,205)
Notional Amount in Effect	50,000
Notional Amount in Effect at Fiscal Year End	50,000
Transaction Date October 2007, Interest Pay Rate 4.642%
Interest rate hedge agreements
Interest Pay Rate (as a percent)	5.015%	[1]
Fair Values	(1,817)
Notional Amount in Effect	75,000
Notional Amount in Effect at Fiscal Year End	75,000
Transaction Date December 2006, Interest Pay Rate 5.015%.
Interest rate hedge agreements
Interest Pay Rate (as a percent)	5.023%	[1]
Fair Values	(1,820)
Notional Amount in Effect	75,000
Notional Amount in Effect at Fiscal Year End	75,000
Transaction Date December 2006, Interest Pay Rate 5.023%
Interest rate hedge agreements
Interest Pay Rate (as a percent)	0.64%	[1]
Fair Values	(291)
Notional Amount in Effect	250,000
Notional Amount in Effect at Fiscal Year End	0
Transaction date December 2011, interest pay rate 0.640%
Interest rate hedge agreements
Interest Pay Rate (as a percent)	0.64%	[1]
Fair Values	(291)
Notional Amount in Effect	250,000
Notional Amount in Effect at Fiscal Year End	0
Transaction date December 2011, interest pay rate 0.640%
Interest rate hedge agreements
Interest Pay Rate (as a percent)	0.644%	[1]
Fair Values	(147)
Notional Amount in Effect	125,000
Notional Amount in Effect at Fiscal Year End	0
Transaction date December 2011, interest pay rate 0.644%
Interest rate hedge agreements
Interest Pay Rate (as a percent)	0.644%	[1]
Fair Values	(147)
Notional Amount in Effect	125,000
Notional Amount in Effect at Fiscal Year End	0
Transaction date December 2011, interest pay rate 0.644%
Interest rate hedge agreements
Interest Pay Rate (as a percent)	0.977%	[1]
Fair Values	(1,802)
Notional Amount in Effect	0
Notional Amount in Effect at Fiscal Year End	250,000
Transaction date December 2011, interest pay rate 0.977%
Interest rate hedge agreements
Interest Pay Rate (as a percent)	0.976%	[1]
Fair Values	(1,799)
Notional Amount in Effect	0
Notional Amount in Effect at Fiscal Year End	$ 250,000

[1] In addition to the interest pay rate, borrowings outstanding under our unsecured senior bank term loans include an applicable margin of 1.20% as of September 30, 2013.