The market value per share of our common stock.

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This amount represents the denominator applied to the principal value of the unsecured debt when applying the conversion rate.

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The component of interest expense representing the noncash expenses charged against earnings in the period to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate caption: Noncash Interest Expense.

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Including the current and noncurrent portions, carrying amount of debt identified as being convertible into another form of financial instrument (typically the entity's common stock) as of the balance sheet date, which originally required full repayment more than twelve months after issuance or greater than the normal operating cycle of the company.

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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 carrying amount of the equity component of convertible debt which may be settled in cash upon conversion.

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The price per share of the conversion feature embedded in the debt instrument.

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The effective interest rate on the liability component of convertible debt instrument which may be settled in cash upon conversion, including partial cash settlement.

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Interest expense related to convertible debt instruments which has been recognized for the period, including the contractual interest coupon and amortization of the debt discount, if any.

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The number of equity instruments that the holder of the debt instrument would receive if the debt was converted to equity.

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Interest rate stated in the contractual debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized.

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Represents the portion of interest incurred in the period on debt arrangements that was charged against earnings, excluding amortization of debt discount (premium) and financing costs.

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Unsecured convertible notes (Details) (USD $)	6 Months Ended			3 Months Ended		6 Months Ended		12 Months Ended			3 Months Ended		6 Months Ended
Unsecured convertible notes (Details) (USD $)	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jun. 30, 2011 8.00% Unsecured Convertible Notes	Jun. 30, 2010 8.00% Unsecured Convertible Notes	Jun. 30, 2011 8.00% Unsecured Convertible Notes	Jun. 30, 2010 8.00% Unsecured Convertible Notes	Dec. 31, 2010 8.00% Unsecured Convertible Notes	Jul. 31, 2010 8.00% Unsecured Convertible Notes	Apr. 30, 2009 8.00% Unsecured Convertible Notes	Jun. 30, 2011 3.70% Unsecured Convertible Notes.	Jun. 30, 2010 3.70% Unsecured Convertible Notes.	Jun. 30, 2011 3.70% Unsecured Convertible Notes.	Jun. 30, 2010 3.70% Unsecured Convertible Notes.	Dec. 31, 2010 3.70% Unsecured Convertible Notes.	Apr. 30, 2009 3.70% Unsecured Convertible Notes.	Jan. 31, 2007 3.70% Unsecured Convertible Notes.
Unsecured convertible notes
Principal amount				$ 250,000	$ 240,000,000	$ 250,000	$ 240,000,000	$ 250,000			$ 205,877,000		$ 205,877,000		$ 301,934,000
Unamortized discount				17,000		17,000		20,000			2,472,000		2,472,000		6,871,000
Net carrying amount of liability component	203,638,000		295,293,000	233,000		233,000		230,000			203,405,000		203,405,000		295,063,000
Carrying amount of equity component				27,000		27,000		27,000			19,616,000		19,616,000		28,769,000
Number of shares on which the aggregate consideration to be delivered on conversion is determined (in shares)				6,038				6,047
Stated interest rate (as a percent)	8.00%	8.00%		8.00%	8.00%	8.00%	8.00%		8.00%	8.00%	3.70%		3.70%		3.70%	3.70%	3.70%
Effective interest rate (as a percent)				11.00%							5.96%
Conversion rate per $1,000 principal value of unsecured convertible notes				$ 41.40		$ 41.40				$ 41.40	$ 117.36		$ 117.36		$ 117.36		$ 117.96
Denominator applied to principal				1,000	1,000	1,000	1,000	1,000		1,000	1,000		1,000		1,000		1,000
Closing stock price of common stock (in dollars per share)											$ 77.42		$ 77.42		$ 73.26
Components of interest cost recognized
Contractual interest				4,000	3,973,000	9,000	8,773,000				1,905,000	3,558,000	3,947,000	7,117,000
Amortization of discount on liability component	2,504,000	5,875,000		2,000	961,000	3,000	2,073,000				1,115,000	1,964,000	2,532,000	3,898,000
Total interest cost				$ 6,000	$ 4,934,000	$ 12,000	$ 10,846,000				$ 3,020,000	$ 5,522,000	$ 6,479,000	$ 11,015,000