Stockholders' Equity	12 Months Ended
Dec. 31, 2017
Stockholders' Equity Note [Abstract]
Stockholders' Equity	Stockholders’ Equity Share Repurchase Program On January 21, 2015, the Company's Board approved a new share repurchase program authorizing the repurchase of up to an aggregate of 20.0 million shares. Share repurchases under the plan may be made through the open market, established plans or privately negotiated transactions in accordance with all applicable securities laws, rules, and regulations. There is no expiration date applicable to the plan. This new stock repurchase program replaced the previous program approved by the Board in February 2010 and canceled the remaining shares outstanding as part of the previous authorization. On May 1, 2017, the Company initiated an accelerated share repurchase program with Barclays Bank PLC. The accelerated share repurchase program is part of the broader share repurchase program previously authorized by the Company's Board on January 21, 2015. Under the accelerated share repurchase program, the Company pre-paid to Barclays Bank PLC, the $50.0 million purchase price for its common stock and, in turn, the Company received an initial delivery of approximately 3.2 million shares of its common stock from Barclays Bank PLC, in the second quarter of 2017, which were retired and recorded as a $40.0 million reduction to stockholders' equity. The remaining $10.0 million of the initial payment was recorded as a reduction to stockholders’ equity as an unsettled forward contract indexed to the Company's stock. The number of shares to be ultimately purchased by the Company was determined based on the volume weighted average price of the common stock during the terms of the transaction, minus an agreed upon discount between the parties. During the fourth quarter of 2017, the accelerated share repurchase program was completed and the Company received an additional 0.8 million shares of its common stock as the final settlement of the accelerated share repurchase program. There were no other repurchases of the Company's common stock during 2017. On October 26, 2015, the Company initiated an accelerated share repurchase program with Citibank, N.A. The accelerated share repurchase program is part of the broader share repurchase program previously authorized by the Company's Board on January 21, 2015. Under the accelerated share repurchase program, the Company pre-paid to Citibank, N.A., the $100.0 million purchase price for its common stock and, in turn, the Company received an initial delivery of approximately 7.8 million shares of its common stock from Citibank, N.A, which were retired and recorded as a $80.0 million reduction to stockholders' equity. The remaining $20.0 million of the initial payment was recorded as a reduction to stockholders’ equity as an unsettled forward contract indexed to the Company's stock. The number of shares to be ultimately purchased by the Company was determined based on the volume weighted average price of the common stock during the terms of the transaction, minus an agreed upon discount between the parties. During the second quarter of 2016, the accelerated share repurchase program was completed and the Company received an additional 0.7 million shares of its common stock as the final settlement of the accelerated share repurchase program. There were no other repurchases of the Company's common stock during 2016. As of December 31, 2017, there remained an outstanding authorization to repurchase approximately 7.4 million shares of the Company’s outstanding common stock under the current share repurchase program. The Company records stock repurchases as a reduction to stockholders’ equity. The Company records a portion of the purchase price of the repurchased shares as an increase to accumulated deficit when the price of the shares repurchased exceeds the average original proceeds per share received from the issuance of common stock. During the year ended December 31, 2017, the cumulative price of $36.6 million was recorded as an increase to accumulated deficit. Convertible Note Hedge Transactions On November 14, 2017 and November 16, 2017, in connection with the 2023 Notes, the Company entered into the Convertible Note Hedge Transactions with respect to the Common Stock, with the Counterparties. The Company paid an aggregate amount of approximately $33.5 million to the Counterparties for the Convertible Note Hedge Transactions. The Convertible Note Hedge Transactions cover, subject to anti-dilution adjustments substantially similar to those in the 2023 Notes, approximately 9.1 million shares of Common Stock, the same number of shares underlying the 2023 Notes, at a strike price that corresponds to the initial conversion price of the 2023 Notes, and are exercisable upon conversion of the 2023 Notes. The Convertible Note Hedge Transactions will expire upon the maturity of the 2023 Notes. The Convertible Note Hedge Transactions are expected generally to reduce the potential dilution to the Common Stock upon conversion of the 2023 Notes and/or offset any cash payments the Company is required to make in excess of the principal amount of the converted 2023 Notes, as the case may be, in the event that the market price per share of the Common Stock, as measured under the terms of the Convertible Note Hedge Transactions, is greater than the strike price of the Convertible Note Hedge Transactions. The Convertible Note Hedge Transactions are separate transactions, entered into by the Company with the Counterparties, and are not part of the terms of the 2023 Notes. Holders of the 2023 Notes will not have any rights with respect to the Convertible Note Hedge Transactions. See Note 10, “Convertible Notes,” for additional details. Warrant Transactions On November 14, 2017 and November 16, 2017, in connection with the 2023 Notes, the Company sold the Warrants to the Counterparties to acquire, collectively, subject to anti-dilution adjustments, approximately 9.1 million shares of the Common Stock at an initial strike price of approximately $23.30 per share, which represents a premium of 60% over the last reported sale price of the Common Stock of $14.56 on November 14, 2017. The Company received aggregate proceeds of approximately $23.2 million from the sale of the Warrants to the Counterparties. The Warrants were sold in private placements to the Counterparties pursuant to an exemption from the registration requirements of the Securities Act afforded by Section 4(a)(2) of the Securities Act. If the market price per share of the Common Stock, as measured under the terms of the Warrants, exceeds the strike price of the Warrants, the Warrants could have a dilutive effect, unless the Company elects, subject to certain conditions, to settle the Warrants in cash. The Warrants are separate transactions, entered into by the Company with the Counterparties, and are not part of the terms of the 2023 Notes. Holders of the 2023 Notes will not have any rights with respect to the Warrants. See Note 10, “Convertible Notes,” for additional details.

Stockholders’ Equity

Share Repurchase Program

On January 21, 2015, the Company's Board approved a new share repurchase program authorizing the repurchase of up to an aggregate of 20.0 million shares. Share repurchases under the plan may be made through the open market, established plans or privately negotiated transactions in accordance with all applicable securities laws, rules, and regulations. There is no expiration date applicable to the plan. This new stock repurchase program replaced the previous program approved by the Board in February 2010 and canceled the remaining shares outstanding as part of the previous authorization.

On May 1, 2017, the Company initiated an accelerated share repurchase program with Barclays Bank PLC. The accelerated share repurchase program is part of the broader share repurchase program previously authorized by the Company's Board on January 21, 2015. Under the accelerated share repurchase program, the Company pre-paid to Barclays Bank PLC, the $50.0 million purchase price for its common stock and, in turn, the Company received an initial delivery of approximately 3.2 million shares of its common stock from Barclays Bank PLC, in the second quarter of 2017, which were retired and recorded as a $40.0 million reduction to stockholders' equity. The remaining $10.0 million of the initial payment was recorded as a reduction to stockholders’ equity as an unsettled forward contract indexed to the Company's stock. The number of shares to be ultimately purchased by the Company was determined based on the volume weighted average price of the common stock during the terms of the transaction, minus an agreed upon discount between the parties. During the fourth quarter of 2017, the accelerated share repurchase program was completed and the Company received an additional 0.8 million shares of its common stock as the final settlement of the accelerated share repurchase program. There were no other repurchases of the Company's common stock during 2017.

On October 26, 2015, the Company initiated an accelerated share repurchase program with Citibank, N.A. The accelerated share repurchase program is part of the broader share repurchase program previously authorized by the Company's Board on January 21, 2015. Under the accelerated share repurchase program, the Company pre-paid to Citibank, N.A., the $100.0 million purchase price for its common stock and, in turn, the Company received an initial delivery of approximately 7.8 million shares of its common stock from Citibank, N.A, which were retired and recorded as a $80.0 million reduction to stockholders' equity. The remaining $20.0 million of the initial payment was recorded as a reduction to stockholders’ equity as an unsettled forward contract indexed to the Company's stock. The number of shares to be ultimately purchased by the Company was determined based on the volume weighted average price of the common stock during the terms of the transaction, minus an agreed upon discount between the parties. During the second quarter of 2016, the accelerated share repurchase program was completed and the Company received an additional 0.7 million shares of its common stock as the final settlement of the accelerated share repurchase program. There were no other repurchases of the Company's common stock during 2016.

As of December 31, 2017, there remained an outstanding authorization to repurchase approximately 7.4 million shares of the Company’s outstanding common stock under the current share repurchase program.

The Company records stock repurchases as a reduction to stockholders’ equity. The Company records a portion of the purchase price of the repurchased shares as an increase to accumulated deficit when the price of the shares repurchased exceeds the average original proceeds per share received from the issuance of common stock. During the year ended December 31, 2017, the cumulative price of $36.6 million was recorded as an increase to accumulated deficit.

Convertible Note Hedge Transactions

On November 14, 2017 and November 16, 2017, in connection with the 2023 Notes, the Company entered into the Convertible Note Hedge Transactions with respect to the Common Stock, with the Counterparties. The Company paid an aggregate amount of approximately $33.5 million to the Counterparties for the Convertible Note Hedge Transactions. The Convertible Note Hedge Transactions cover, subject to anti-dilution adjustments substantially similar to those in the 2023 Notes, approximately 9.1 million shares of Common Stock, the same number of shares underlying the 2023 Notes, at a strike price that corresponds to the initial conversion price of the 2023 Notes, and are exercisable upon conversion of the 2023 Notes. The Convertible Note Hedge Transactions will expire upon the maturity of the 2023 Notes.

The Convertible Note Hedge Transactions are expected generally to reduce the potential dilution to the Common Stock upon conversion of the 2023 Notes and/or offset any cash payments the Company is required to make in excess of the principal amount of the converted 2023 Notes, as the case may be, in the event that the market price per share of the Common Stock, as measured under the terms of the Convertible Note Hedge Transactions, is greater than the strike price of the Convertible Note Hedge Transactions.

The Convertible Note Hedge Transactions are separate transactions, entered into by the Company with the Counterparties, and are not part of the terms of the 2023 Notes. Holders of the 2023 Notes will not have any rights with respect to the Convertible Note Hedge Transactions. See Note 10, “Convertible Notes,” for additional details.

Warrant Transactions

On November 14, 2017 and November 16, 2017, in connection with the 2023 Notes, the Company sold the Warrants to the Counterparties to acquire, collectively, subject to anti-dilution adjustments, approximately 9.1 million shares of the Common Stock at an initial strike price of approximately $23.30 per share, which represents a premium of 60% over the last reported sale price of the Common Stock of $14.56 on November 14, 2017. The Company received aggregate proceeds of approximately $23.2 million from the sale of the Warrants to the Counterparties. The Warrants were sold in private placements to the Counterparties pursuant to an exemption from the registration requirements of the Securities Act afforded by Section 4(a)(2) of the Securities Act.

If the market price per share of the Common Stock, as measured under the terms of the Warrants, exceeds the strike price of the Warrants, the Warrants could have a dilutive effect, unless the Company elects, subject to certain conditions, to settle the Warrants in cash.

The Warrants are separate transactions, entered into by the Company with the Counterparties, and are not part of the terms of the 2023 Notes. Holders of the 2023 Notes will not have any rights with respect to the Warrants. See Note 10, “Convertible Notes,” for additional details.