Debt (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	0 Months Ended	3 Months Ended		0 Months Ended
	Feb. 13, 2014	Mar. 31, 2015	Mar. 31, 2014	Feb. 14, 2014	Dec. 31, 2014	Feb. 28, 2014	Feb. 11, 2014
Liability component:
Principal		$ 200,000,000
Less: debt discount, net		(30,910,000)
Net carrying amount		169,090,000			167,441,000
Debt Instrument, Fair Value Disclosure		433,500,000
Equity component		38,188,000
Interest rate (as a percent)		2.50%			2.50%
Sale price of common stock (in dollars per share)		$ 44.00
Total interest expense recognized
Total interest expense		10,367,000	1,476,000
Convertible Bond Hedge
Common stock covered under convertible bond hedge (in shares)	7.4
Exercise price (in dollars per unit)	$ 27.09
Purchase of convertible bond hedges, net of tax			39,760,000
Warrant Transactions
Number of shares of common stock called by warrants						7.4
Exercise price (in dollars per share)						$ 34.12
Exercise price above last reported sale price of common stock (as a percent)						70.00%
Proceeds from issuance of warrants			25,620,000
Convertible Debt
Liability component:
Net carrying amount		169,100,000
Net proceeds from issuance of convertible debt				193,300,000
Aggregate principal amount of notes issued				200,000,000
Amount pertaining to full exercise of over-allotment of debt by underwriters				25,000,000
Fees and expenses				6,700,000
Proceeds used to pay the cost of the bond hedges (after such cost was partially offset by proceeds from the sale of warrants)				14,100,000
Interest rate (as a percent)		2.50%
Initial conversion rate of common stock per $1000 of principal amount of Notes (in shares)		36.9079
Principal amount used for debt instrument conversion ratio		1,000
Initial conversion price of convertible notes into common stock (in dollars per share)		$ 27.09
Sale price of common stock (in dollars per share)							$ 20.07
Debt issuance costs allocated to equity component				1,300,000
Debt issuance costs allocated to the liability component				5,400,000
Expected life of the debt		5 years
Effective interest rate on liability component (as a percent)		7.23%
Total interest expense recognized
Contractual interest expense		1,250,000
Amortization of debt issuance costs		234,000
Amortization of debt discount		1,649,000
Total interest expense		3,133,000
Convertible Debt | Debt Instrument Convertible Covenant One [Member]
Liability component:
Number of days during 30 consecutive trading days in which the closing price of the entity's common stock must exceed or equal the conversion price for the notes to be convertible		20
Number of consecutive trading days during which the closing price of the entity's common stock must exceed or equal the conversion price for at least 20 days in order for the notes to be convertible		30 days
Percentage of the closing sales price of the entity's common stock that the conversion price must exceed or be equal in order for the notes to be convertible		130.00%
Convertible Debt | Debt Instrument Convertible Covenant Two [Member]
Liability component:
Principal amount used for debt instrument conversion ratio		1,000
Number of consecutive business days 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
Convertible Debt | Debt Instrument Convertible Covenant Two [Member] | Maximum
Liability component:
Percentage of product of the last reported sale price of the entity's common stock and the conversion rate of convertible debt instruments		98.00%
Convertible Debt | Debt Instrument Convertible Covenant Three [Member]
Liability component:
Principal amount used for debt instrument conversion ratio		$ 1,000