Convertible Notes (Details) (USD $)			1 Months Ended		3 Months Ended	12 Months Ended					1 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Jul. 31, 2009 5% Convertible Senior Notes due 2014	Jun. 30, 2009 5% Convertible Senior Notes due 2014	Dec. 31, 2010 5% Convertible Senior Notes due 2014	Dec. 31, 2012 5% Convertible Senior Notes due 2014	Dec. 31, 2012 5% Convertible Senior Notes due 2014	Dec. 31, 2011 5% Convertible Senior Notes due 2014	Dec. 31, 2010 5% Convertible Senior Notes due 2014	Jun. 29, 2009 5% Convertible Senior Notes due 2014	Feb. 28, 2010 Zero Coupon Convertible Senior Notes due 2010	Feb. 28, 2005 Zero Coupon Convertible Senior Notes due 2010	Dec. 31, 2012 Zero Coupon Convertible Senior Notes due 2010
Convertible Notes
Total principal amount of convertible notes	$ 172,500,000	$ 172,500,000				$ 172,500,000	$ 172,500,000	$ 172,500,000
Unamortized discount	(24,900,000)	(39,000,000)				(24,944,000)	(24,944,000)	(39,007,000)
Total convertible notes						147,556,000	147,556,000	133,493,000
Total long-term convertible notes	147,556,000	133,493,000				147,556,000	147,556,000	133,493,000
Additional borrowings on debt instrument			22,500,000	150,000,000								300,000,000
Convertible notes stated interest rate (as a percent)					5.00%	5.00%	5.00%	5.00%	5.00%	5.00%
Liability component of convertible senior notes principal issued			14,300,000	92,400,000
Equity component of convertible senior notes principal issued			8,200,000	57,600,000
Amortization period of unamortized discount (in years)										5 years
Periodic interest payment							8,600,000	8,600,000	8,600,000
Issuance cost					5,100,000
Convertible debt issuance costs - liability component					3,200,000
Amortization period of debt issuance costs (in years)						5 years	5 years
Convertible debt issuance costs - equity component					1,900,000
Conversion rate, number of shares to be issued per $1000 of principal (in shares)						51.8	51.8						37.2585
Principal amount of notes used as the denominator to determine number of shares converted into notes						1,000	1,000						1,000
Initial conversion price of notes (in dollars per share)						$ 19.31	$ 19.31						$ 26.84
Number of trading days within 30 consecutive trading days in which the closing price of the entity's common stock must exceed the conversion price for the notes to be redeemable, minimum						20 days	20 days
Number of consecutive trading days during which the closing price of the entity's common stock must exceed the conversion price for at least 20 days in order for the notes to be redeemable						30 days	30 days
Percentage of the closing sales price of the entity's common stock that the conversion price must exceed in order for the notes to be convertible						130.00%	130.00%
Number of business days immediately after any ten consecutive trading day period during the note measurement period						5 days	5 days
Number of consecutive trading days before the five business days during the debt instrument measurement period						10 days	10 days
Denomination of the principal amount of notes used to calculate the percent of trading price during the debt instrument measurement period						1,000	1,000
Maximum conversion price as a percentage of closing stock price						98.00%	98.00%
Number of trading days conversion reference period						20 days	20 days
Percentage of face amount of debt instrument redeemable at the company's option						100.00%	100.00%
Number of trading days within 30 consecutive trading days in which the closing price of the entity's common stock must exceed the conversion price for the notes to be redeemable at the company's option, minimum						20 days	20 days
Number of consecutive trading days during which the closing price of the entity's common stock must exceed the conversion price for at least 20 days in order for the notes to be redeemable at the company's option						30 days	30 days
Percentage of the closing sales price of the entity's common stock that the conversion price must exceed in order for the notes to be redeemable at the company's option						130.00%	130.00%
Percentage of face amount which may be redeemed by the debt holder for cash upon occurrence of a fundamental change						100.00%	100.00%
Maximum increase in conversion rate per $1000 principal amount upon occurrence of fundamental change						15.5401	1,000
Additional paid-in capital						63,900,000	63,900,000	63,900,000,000
Events of default
Period of default in payment of interest (in days)						30 days	30 days
Period of default to comply with other agreements (in days)						60 days	60 days
Minimum percentage of aggregate outstanding principal required for default event with other agreements						25.00%	25.00%
Minimum principal amount of debt nonpayment required for debt default to occur						30,000,000	30,000,000
Period of nonpayment of principal amount required for debt default to occur (in days)						10 days	10 days
Minimum percentage of aggregate outstanding principal required for nonpayment of debt default to occur						25.00%	25.00%
Minimum percentage of aggregate outstanding principal required for immediate payment declaration to occur						25.00%	25.00%
Repayment of debt upon maturity											$ 137,000,000