Fair value measurements (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Assets:
Marketable securities	$ 6,563	$ 6,235
Fair value measured on recurring basis \| Fair Value
Assets:
Marketable securities	6,563	6,235
Liabilities:
Interest rate swap agreements	25,336	32,980
Fair value measured on recurring basis \| Quoted Prices in Active Markets for Identical Assets and Liabilities
Assets:
Marketable securities	6,563	6,235
Fair value measured on recurring basis \| "Significant Other Observable Inputs"
Liabilities:
Interest rate swap agreements	$ 25,336	$ 32,980

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

+ References+ Details