Interest rate swap agreements (Details 2) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Interest rate swap agreements
Interest rate cash flow hedge gain (loss) expected to be reclassified to interest expense in next fiscal year				$ 16,500,000
Interest rate hedge agreements
Fair Values				(25,336,000)	(33,000,000)
Notional Amount in Effect	500,000,000	950,000,000	1,300,000,000	1,400,000,000
Unsecured Senior Line of Credit and Bank Term Loans | Minimum
Interest rate hedge agreements
Applicable margin (as a percent)				1.20%
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%
Fair Values				(3,614,000)
Notional Amount in Effect		50,000,000	50,000,000	50,000,000
Transaction Date October 2007, Interest Pay Rate 4.642%
Interest rate hedge agreements
Interest Pay Rate (as a percent)				4.642%
Fair Values				(2,251,000)
Notional Amount in Effect			50,000,000	50,000,000
Transaction Date October 2007, Interest Pay Rate 4.622%
Interest rate hedge agreements
Interest Pay Rate (as a percent)				4.622%
Fair Values				(555,000)
Notional Amount in Effect			25,000,000	25,000,000
Transaction Date October 2007, Interest Rate 4.625%.
Interest rate hedge agreements
Interest Pay Rate (as a percent)				4.625%
Fair Values				(555,000)
Notional Amount in Effect			25,000,000	25,000,000
Transaction Date December 2006, Interest Pay Rate 5.015%.
Interest rate hedge agreements
Interest Pay Rate (as a percent)				5.015%
Fair Values				(5,449,000)
Notional Amount in Effect		75,000,000	75,000,000	75,000,000
Transaction Date December 2006, Interest Pay Rate 5.023%
Interest rate hedge agreements
Interest Pay Rate (as a percent)				5.023%
Fair Values				(5,458,000)
Notional Amount in Effect		75,000,000	75,000,000	75,000,000
Transaction Date December 2006, Interest Pay Rate 5.015%
Interest rate hedge agreements
Interest Pay Rate (as a percent)				5.015%
Fair Values				(440,000)
Notional Amount in Effect				100,000,000
Transaction date December 2011, interest pay rate 0.480%
Interest rate hedge agreements
Interest Pay Rate (as a percent)				0.48%
Fair Values				(180,000)
Notional Amount in Effect			250,000,000	250,000,000
Transaction date December 2011, interest pay rate 0.480%
Interest rate hedge agreements
Interest Pay Rate (as a percent)				0.48%
Fair Values				(180,000)
Notional Amount in Effect			250,000,000	250,000,000
Transaction date December 2011, interest pay rate 0.480%
Interest rate hedge agreements
Interest Pay Rate (as a percent)				0.48%
Fair Values				(90,000)
Notional Amount in Effect			125,000,000	125,000,000
Transaction date December 2011, Interest Pay Rate 0.480%
Interest rate hedge agreements
Interest Pay Rate (as a percent)				0.48%
Fair Values				(90,000)
Notional Amount in Effect			125,000,000	125,000,000
Transaction date December 2011, Interest Pay Rate 0.495%
Interest rate hedge agreements
Interest Pay Rate (as a percent)				0.495%
Fair Values				(95,000)
Notional Amount in Effect			125,000,000	125,000,000
Transaction date December 2011, Interest Pay Rate 0.508%
Interest rate hedge agreements
Interest Pay Rate (as a percent)				0.508%
Fair Values				(99,000)
Notional Amount in Effect			125,000,000	125,000,000
Transaction date December 2011, interest pay rate 0.640%
Interest rate hedge agreements
Interest Pay Rate (as a percent)				0.64%
Fair Values				(1,041,000)
Notional Amount in Effect		250,000,000
Transaction date December 2011, interest pay rate 0.640%
Interest rate hedge agreements
Interest Pay Rate (as a percent)				0.64%
Fair Values				(1,041,000)
Notional Amount in Effect		250,000,000
Transaction date December 2011, interest pay rate 0.644%
Interest rate hedge agreements
Interest Pay Rate (as a percent)				0.644%
Fair Values				(526,000)
Notional Amount in Effect		125,000,000
Transaction date December 2011, interest pay rate 0.644%
Interest rate hedge agreements
Interest Pay Rate (as a percent)				0.644%
Fair Values				(526,000)
Notional Amount in Effect		125,000,000
Transaction date December 2011, interest pay rate 0.977%
Interest rate hedge agreements
Interest Pay Rate (as a percent)				0.977%
Fair Values				(1,574,000)
Notional Amount in Effect	250,000,000
Transaction date December 2011, interest pay rate 0.976%
Interest rate hedge agreements
Interest Pay Rate (as a percent)				0.976%
Fair Values				(1,572,000)
Notional Amount in Effect	$ 250,000,000