Interest rate swap agreements Outstanding interest rate swap (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Interest rate hedge agreements
Fair Values	$ (12,547)		$ (20,700)
Notional Amount in Effect	1,000,000
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)	1.50%
Unsecured Senior Line of Credit and Bank Term Loans | Maximum
Interest rate hedge agreements
Applicable margin (as a percent)	1.75%
Transaction Date December 2006, Interest Pay Rate 4.990%
Interest rate hedge agreements
Interest Pay Rate (as a percent)	4.99%	[1]
Fair Values	(1,804)
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)	4.642%	[1]
Fair Values	(563)
Notional Amount in Effect	50,000
Notional Amount in Effect at Fiscal Year End	0
Transaction Date December 2006, Interest Pay Rate 5.015%.
Interest rate hedge agreements
Interest Pay Rate (as a percent)	5.015%	[1]
Fair Values	(2,721)
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)	5.023%	[1]
Fair Values	(2,725)
Notional Amount in Effect	75,000
Notional Amount in Effect at Fiscal Year End	75,000
Transaction date December 2011, interest pay rate 0.640%
Interest rate hedge agreements
Interest Pay Rate (as a percent)	0.64%	[1]
Fair Values	(553)
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	(553)
Notional Amount in Effect	250,000
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	(279)
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	(279)
Notional Amount in Effect	125,000
Notional Amount in Effect at Fiscal Year End	0
Transaction date December 2011, interest pay rate 0.977%
Interest rate hedge agreements
Interest Pay Rate (as a percent)	0.977%	[1]
Fair Values	(1,536)
Notional Amount in Effect	0
Notional Amount in Effect at Fiscal Year End	250,000
Transaction date December 2011, interest pay rate 0.976%
Interest rate hedge agreements
Interest Pay Rate (as a percent)	0.976%	[1]
Fair Values	(1,534)
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 currently ranging from 1.50% to 1.75% as of June 30, 2013.