Carrying value as of the balance sheet date of long-term debt (with maturities initially due after one year or beyond the operating cycle if longer) identified as Convertible Notes Payable, excluding current portion. Convertible Notes Payable is a written promise to pay a note which can be exchanged for a specified amount of another, related security, at the option of the issuer and the holder.

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Including current and noncurrent portions, aggregate carrying amount of long-term borrowings as of the balance sheet date before deducting unamortized discount or premiums (if any). May include notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, which had initial maturities beyond one year or beyond the normal operating cycle, if longer.

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The stated principal amount of the debt instrument at time of issuance, which may vary from the carrying amount because of unamortized premium or discount.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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Senior convertible notes (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Senior convertible notes due december 15 2013 [Member]	Dec. 31, 2011 Senior convertible notes due december 15 2013 [Member]	Dec. 31, 2006 Senior convertible notes due december 15 2013 [Member]	Sep. 30, 2012 Senior convertible notes due august 15 2015 [Member]	Dec. 31, 2011 Senior convertible notes due august 15 2015 [Member]	Aug. 31, 2010 Senior convertible notes due august 15 2015 [Member]
Summary of Senior convertible notes
Principal amount			$ 115,000	$ 115,000	$ 115,000	$ 100,000	$ 100,000	$ 100,000
Unaccreted debt issuance expense			(705)	(1,140)		(2,622)	(3,218)
Net carrying amount			114,295	113,860		97,378	96,782
Senior convertible notes	$ 211,673	$ 210,642