Debt - Convertible Debt (Details)	1 Months Ended	3 Months Ended	12 Months Ended
	Jun. 30, 2011 USD ($) $ / shares	Jun. 30, 2011 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares shares
Convertible notes payable
Debt Instrument [Line Items]
Principal amount			$ 316,300,000
Initial conversion rate (in shares per $1,000 of principal)		34.1006
Conversion rate per value of notes			$ 1,000
Equivalent initial conversion price (in dollars per share) | $ / shares	$ 29.325	$ 29.325
Number of trading days for pricing			20 days
Number of consecutive trading days			30 days
Percentage minimum of applicable conversion price (in hundredths)			130.00%
Number of consecutive trading days less than 98% of test period			5 days
Applicable percentage rate for five day consecutive trading days (in hundredths)			98.00%
Imputed interest rate	7.50%	7.50%
Expected life of convertible debt		7 years
Net cost of hedging transaction	$ 31,900,000	$ 31,900,000
Convertible notes payable | Convertible Senior Notes Due June 2018
Debt Instrument [Line Items]
Principal amount	$ 316,300,000	$ 316,300,000
Interest rate	2.75%	2.75%
Current portion of convertible debt			$ 309,900,000
Proceeds from debt financing	$ 308,200,000
Warrants
Debt Instrument [Line Items]
Number of shares of common stock covered by hedging transactions | shares			10,784,315
Number of warrants to acquire common stock | shares			10,784,315
Strike price of warrants (in dollars per share) | $ / shares			$ 40.25