Fair value measurements (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Assets:
Marketable securities	$ 3,435	$ 2,709
Fair value measured on recurring basis | Fair Value
Assets:
Marketable securities	3,435	2,709
Liabilities:
Interest rate swap agreements	12,547	20,661
Fair value measured on recurring basis | Quoted Prices in Active Markets for Identical Assets and Liabilities
Assets:
Marketable securities	3,435	2,709
Liabilities:
Interest rate swap agreements	0	0
Fair value measured on recurring basis | Significant Other Observable Inputs
Assets:
Marketable securities	0	0
Liabilities:
Interest rate swap agreements	12,547	20,661
Fair value measured on recurring basis | Significant Unobservable Inputs
Assets:
Marketable securities	0	0
Liabilities:
Interest rate swap agreements	$ 0	$ 0