Fair value of financial instruments (Details) (Fair value measured on a recurring basis, USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair value measurement at reporting date using: Quoted prices in active markets for identical assets (Level 1)
Fair value of financial instruments
Cash and cash equivalents	$ 265,814	$ 224,057
Total assets	265,814	224,057
Fair value measurement at reporting date using: Significant other observable inputs (Level 2)
Fair value of financial instruments
Mortgage backed securities	4,365	4,361
Total assets	4,365	4,361
Fair Value
Fair value of financial instruments
Cash and cash equivalents	265,814	224,057
Mortgage backed securities	4,365	4,361
Total assets	$ 270,179	$ 228,418

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 includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Company may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months.

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