Long-Term Obligations (Details) (USD $)	0 Months Ended	12 Months Ended
	Jan. 31, 2008	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Information related to long-term obligations
Convertible subordinated notes		$ 172,500,000	$ 172,500,000
Convertible subordinated notes
Debt disclosures
Loan amount	172,500,000
Proceeds from issuance of debt, net of issuance costs	166,700,000
Interest rate (as a percent)		3.00%
Conversion rate, number of shares to be issued per $1000 of principal amount of notes		38.6548
Principal amount used for debt instrument conversion ratio		1,000
Initial conversion price of convertible notes into common stock (in dollars per share)		$ 25.87
Unamortized debt issuance costs		1,700,000
Amortization expense		$ 800,000	$ 800,000	$ 800,000
Percentage of principal amount at which the entity may redeem some or all notes		100.00%
Percentage of closing sales price of the entity's common stock the conversion price must exceed in order for the notes to be convertible		130.00%
Number of 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		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