Stockholder's Equity	12 Months Ended
Dec. 31, 2018
Equity [Abstract]
Stockholder's Equity	Stockholders’ Equity Pursuant to the Company’s Certificate of Incorporation, the Board of Directors may establish one or more series of preferred stock. Other than the Series A Convertible Perpetual Preferred Stock, par value $0.001 per share (the “Series A Preferred Stock”), no shares of preferred stock are currently outstanding. Series A Convertible Perpetual Preferred Stock and Warrants In 2011, the Company issued to certain investors, for $75 million in cash: (i) an aggregate of 75,000 shares of the Series A Preferred Stock with an initial liquidation preference of $1,000 per share, which are convertible into shares of Company common stock at a conversion price of $7.00 per common share (subject to customary anti-dilution adjustments), and (ii) warrants exercisable for shares of Company common stock at an initial exercise price of $7.00 per common share (subject to customary anti-dilution adjustments) (the “Warrants”). As of December 31, 2018, the outstanding Series A Preferred Stock is convertible into 10 million shares of Company common stock and there are outstanding Warrants exercisable for an aggregate of 10 million shares of Company common stock. The Series A Preferred Stock ranks, with respect to dividend rights and rights upon liquidation, winding-up or dissolution of the Company, senior to the Company’s common stock and to each other class or series of stock of the Company (including any series of preferred stock) the terms of which do not expressly provide that such class or series ranks senior to or pari passu with the Series A Preferred Stock. The Series A Preferred Stock pays quarterly cash dividends equal to the greater of: (i) the “as-converted” dividends on the underlying Company common stock for the relevant quarter, and: (ii) 4% of the then-applicable liquidation preference per annum. The Series A Preferred Stock is not redeemable or subject to any required offer to purchase and votes together with the Company’s common stock on an “as-converted” basis on all matters, except as otherwise required by law, and separately as a class with respect to certain matters implicating the rights of holders of shares of Series A Preferred Stock. Equity Offering and Forward Sale Agreements In July 2017, the Company completed a registered underwritten offering of 11 million shares of its common stock at a public offering price of $60.50 per share (the “Offering”). Of the 11 million shares of common stock, five million shares were offered directly by the Company and six million shares were offered in connection with forward sale agreements (the “Forward Sale Agreements”) described below. The Offering closed on July 25, 2017. The Company received proceeds of $290 million ($288 million net of fees and expenses) from the sale of five million shares of common stock in the Offering. The Company used the net proceeds of the shares issued and sold by the Company in the Offering for general corporate purposes. In connection with the Offering, the Company entered into separate Forward Sale Agreements with Morgan Stanley & Co. LLC and JPMorgan Chase Bank, National Association, London Branch (the “Forward Counterparties”) pursuant to which the Company agreed to sell, and each Forward Counterparty agreed to purchase, three million shares of the Company’s common stock (or six million shares of the Company common stock in the aggregate) subject to the terms and conditions of the Forward Sale Agreements, including the Company’s right to elect cash settlement or net share settlement. The initial forward price under each of the Forward Sale Agreements is $58.08 per share (which was the public offering price of the Company’s common stock for the primary offering of the five million shares described above, less the underwriting discount) and was subject to certain adjustments pursuant to the terms of the Forward Sale Agreements. Consistent with the Company’s strategy to grow its business in part through acquisitions, the Company entered into the Forward Sale Agreements to provide additional available cash for such acquisitions, among other general corporate purposes. In July 2018, the Company physically settled the forwards in full by delivering six million shares of common stock to the Forward Counterparties for net cash proceeds to the Company of $349 million. As a part of its ordinary course treasury management activities, the Company applied these net cash proceeds to the repayment of the Senior Notes due 2022 as described above. Share Repurchases On December 14, 2018, the Company’s Board of Directors authorized share repurchases of up to $1 billion of the Company’s common stock. The repurchase authorization permits the Company to repurchase shares in both open market and private repurchase transactions, with the timing and number of shares repurchased dependent on a variety of factors, including price, general business and market conditions, alternative investment opportunities and funding considerations. Through December 31, 2018, based on the settlement date, the Company purchased and retired 10 million shares of its common stock having an aggregate value of $536 million at an average price of $53.46 per share. In January and February 2019, based on the settlement date, the Company purchased and retired 8 million shares of its common stock having an aggregate value of $464 million at an average price of $59.47 per share, which completed the authorized repurchase program. The share repurchases were funded by the unsecured credit facility and available cash.

Stockholders’ Equity

Pursuant to the Company’s Certificate of Incorporation, the Board of Directors may establish one or more series of preferred stock. Other than the Series A Convertible Perpetual Preferred Stock, par value $0.001 per share (the “Series A Preferred Stock”), no shares of preferred stock are currently outstanding.

Series A Convertible Perpetual Preferred Stock and Warrants

In 2011, the Company issued to certain investors, for $75 million in cash: (i) an aggregate of 75,000 shares of the Series A Preferred Stock with an initial liquidation preference of $1,000 per share, which are convertible into shares of Company common stock at a conversion price of $7.00 per common share (subject to customary anti-dilution adjustments), and (ii) warrants exercisable for shares of Company common stock at an initial exercise price of $7.00 per common share (subject to customary anti-dilution adjustments) (the “Warrants”). As of December 31, 2018, the outstanding Series A Preferred Stock is convertible into 10 million shares of Company common stock and there are outstanding Warrants exercisable for an aggregate of 10 million shares of Company common stock. The Series A Preferred Stock ranks, with respect to dividend rights and rights upon liquidation, winding-up or dissolution of the Company, senior to the Company’s common stock and to each other class or series of stock of the Company (including any series of preferred stock) the terms of which do not expressly provide that such class or series ranks senior to or pari passu with the Series A Preferred Stock. The Series A Preferred Stock pays quarterly cash dividends equal to the greater of: (i) the “as-converted” dividends on the underlying Company common stock for the relevant quarter, and: (ii) 4% of the then-applicable liquidation preference per annum. The Series A Preferred Stock is not redeemable or subject to any required offer to purchase and votes together with the Company’s common stock on an “as-converted” basis on all matters, except as otherwise required by law, and separately as a class with respect to certain matters implicating the rights of holders of shares of Series A Preferred Stock.

Equity Offering and Forward Sale Agreements

In July 2017, the Company completed a registered underwritten offering of 11 million shares of its common stock at a public offering price of $60.50 per share (the “Offering”). Of the 11 million shares of common stock, five million shares were offered directly by the Company and six million shares were offered in connection with forward sale agreements (the “Forward Sale Agreements”) described below. The Offering closed on July 25, 2017.

The Company received proceeds of $290 million ($288 million net of fees and expenses) from the sale of five million shares of common stock in the Offering. The Company used the net proceeds of the shares issued and sold by the Company in the Offering for general corporate purposes.

In connection with the Offering, the Company entered into separate Forward Sale Agreements with Morgan Stanley & Co. LLC and JPMorgan Chase Bank, National Association, London Branch (the “Forward Counterparties”) pursuant to which the Company agreed to sell, and each Forward Counterparty agreed to purchase, three million shares of the Company’s common stock (or six million shares of the Company common stock in the aggregate) subject to the terms and conditions of the Forward Sale Agreements, including the Company’s right to elect cash settlement or net share settlement. The initial forward price under each of the Forward Sale Agreements is $58.08 per share (which was the public offering price of the Company’s common stock for the primary offering of the five million shares described above, less the underwriting discount) and was subject to certain adjustments pursuant to the terms of the Forward Sale Agreements. Consistent with the Company’s strategy to grow its business in part through acquisitions, the Company entered into the Forward Sale Agreements to provide additional available cash for such acquisitions, among other general corporate purposes. In July 2018, the Company physically settled the forwards in full by delivering six million shares of common stock to the Forward Counterparties for net cash proceeds to the Company of $349 million. As a part of its ordinary course treasury management activities, the Company applied these net cash proceeds to the repayment of the Senior Notes due 2022 as described above.

Share Repurchases

On December 14, 2018, the Company’s Board of Directors authorized share repurchases of up to $1 billion of the Company’s common stock. The repurchase authorization permits the Company to repurchase shares in both open market and private repurchase transactions, with the timing and number of shares repurchased dependent on a variety of factors, including price, general business and market conditions, alternative investment opportunities and funding considerations. Through December 31, 2018, based on the settlement date, the Company purchased and retired 10 million shares of its common stock having an aggregate value of $536 million at an average price of $53.46 per share. In January and February 2019, based on the settlement date, the Company purchased and retired 8 million shares of its common stock having an aggregate value of $464 million at an average price of $59.47 per share, which completed the authorized repurchase program. The share repurchases were funded by the unsecured credit facility and available cash.