Debt - Narrative (Details) $ / shares in Units, shares in Millions	1 Months Ended		12 Months Ended
	May 31, 2018 USD ($) d $ / shares shares	Feb. 28, 2014 USD ($) d $ / shares shares	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Fair value of convertible senior notes			$ 686,400,000
Closing stock price (in dollars per share) | $ / shares			$ 61.08
Repurchases of common stock		$ 62,000,000	$ 750,000,000	$ 361,194,000	$ 373,794,000
Payments for purchase of convertible note hedge and warrant transactions		23,300,000
Payments for note hedge transactions		$ 101,300,000	261,740,000	0	0
Note hedge shares outstanding (in shares) | shares		7.7
Warrants outstanding (in shares) | shares		7.7
Warrant strike price (in dollars per share) | $ / shares		$ 104.49
Proceeds from sale of warrants		$ 78,000,000	119,945,000	0	$ 0
Convertible Debt | 2025 Notes
Debt Instrument [Line Items]
Debt issued	$ 1,150,000,000.0		1,150,000,000
Interest rate	0.125%
Threshold trading days exceeding price | d	20
Threshold consecutive trading days exceeding price | d	30
Threshold greater than percentage of stock price trigger	130.00%
Threshold trading days not exceeding price	5 years
Threshold consecutive trading days not exceeding price	5 years
Principal amount per conversion	$ 1,000
Threshold less than percentage of stock price trigger	98.00%
Conversion rate	0.010515
Conversion price (in dollars per share) | $ / shares	$ 95.10
Potential conversion shares of convertible debt (in shares) | shares	12.1
Fair value of convertible senior notes			$ 1,049,600,000
Closing stock price (in dollars per share) | $ / shares			$ 61.08
Repurchases of common stock	$ 46,200,000
Payments for purchase of convertible note hedge and warrant transactions	141,800,000
Equity component:			$ 285,225,000
Payments for note hedge transactions	$ 261,700,000
Warrants outstanding (in shares) | shares	12.1
Warrant strike price (in dollars per share) | $ / shares	$ 149.18
Proceeds from sale of warrants	$ 119,900,000
Effective interest rate			4.26%
Convertible Debt | 2019 Notes
Debt Instrument [Line Items]
Debt issued		$ 690,000,000	$ 690,000,000	690,000,000
Threshold trading days exceeding price | d		20
Threshold consecutive trading days exceeding price | d		30
Threshold greater than percentage of stock price trigger		130.00%
Threshold trading days not exceeding price		5 days
Threshold consecutive trading days not exceeding price		5 days
Principal amount per conversion		$ 1,000
Threshold less than percentage of stock price trigger		98.00%
Conversion rate		0.0111651
Conversion price (in dollars per share) | $ / shares		$ 89.56
Potential conversion shares of convertible debt (in shares) | shares		7.7
Equity component:			$ 101,276,000	$ 101,276,000
Effective interest rate			3.20%