Interest rate swap agreements (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Interest rate swap agreements
The percentage of effectiveness of interest rate swap agreements	100.00%	100.00%	100.00%	100.00%
Cash flow hedges
Interest rate hedge agreements
Unrealized loss recognized in other comprehensive loss	3,091	5,555	7,164	5,255
Cash flow hedges | Interest rate swaps
Interest rate hedge agreements
Unrealized loss recognized in other comprehensive loss	3,091	5,555	7,164	(5,255)
Cash flow hedges | Interest expense
Interest rate hedge agreements
Loss reclassified from accumulated other comprehensive income to earnings	5,895	5,302	11,670	10,741
Cash flow hedges | Interest expense | Interest rate swaps
Interest rate hedge agreements
Loss reclassified from accumulated other comprehensive income to earnings	5,895	5,302	11,670	10,741