Convertible Senior Notes (Details) (USD $)	0 Months Ended	1 Months Ended	3 Months Ended	9 Months Ended
	Aug. 12, 2013	Aug. 31, 2013	Sep. 30, 2013	Sep. 30, 2013 item
Convertible Senior Notes
Initial net proceeds				$ 96,608,000
Shares purchased				84,054
Aggregate purchase price				1,000,000
Components of Notes
Net carrying amount			81,670,000	81,670,000
Notes
Convertible Senior Notes
Debt instrument, stated interest rate (as a percent)		2.75%
Principal amount		100,000,000
Initial conversion rate of common stock (in shares)				0.0532907
Initial conversion price of Notes into common stock (in dollars per share)			$ 18.77	$ 18.77
Percentage of principal amount of the notes, plus any accrued and unpaid interest, at which the holder of the Notes may require the entity to repurchase the debt instrument if the entity undergoes certain fundamental changes				100.00%
Number of days within 30 consecutive trading days in which the closing price of the entity's common stock must exceed 130% of the conversion price as a condition for conversion of Notes				20
Number of consecutive trading days during which the closing price of the entity's common stock must exceed 130% of the conversion price for at least 20 days as a condition for conversion of Notes				30 days
Percentage of the conversion price that the closing sales price of the entity's common stock must exceed as a condition for conversion of Notes				130.00%
Number of consecutive business days immediately after any five consecutive trading day period during the note measurement period				5 days
Number of consecutive trading days before five consecutive business days during the note measurement period				5 days
Principal amount used for debt instrument conversion ratio			1,000	1,000
Issuance costs attributable to the liability component		2,800,000
Issuance costs attributable to the equity component		600,000	600,000	600,000
Initial net proceeds		96,600,000
Shares purchased	1,798,561
Aggregate purchase price	25,000,000
Components of Notes
Principal			100,000,000	100,000,000
Unamortized debt discount			(18,330,000)	(18,330,000)
Net carrying amount			81,670,000	81,670,000
Carrying amount of the equity component			18,138,000	18,138,000
Remaining life				59 months
Total interest expense recognized related to the Notes
Contractual interest expense			390,000	390,000
Interest cost related to amortization of debt issuance costs			70,000	70,000
Interest cost related to amortization of the debt discount			$ 446,000	$ 446,000
Effective interest rate of the liability component (as a percent)			7.90%	7.90%
Notes | Maximum
Convertible Senior Notes
Percentage of the trading price to the product of the last reported sale price of the entity's common stock and the conversion rate				98.00%