Stock-Based Compensation - Stock Options (Details) - Outstanding stock options $ / shares in Units, $ in Millions		9 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares
Shares
Outstanding at the beginning of the period (in shares) | shares		6,908,313
Granted (in shares) | shares		1,663,879
Exercised (in shares) | shares		(193,895)
Cancelled (in shares) | shares		(668,765)
Outstanding at the end of the period (in shares) | shares	7,709,532	7,709,532
Vested at the end of the period (in shares) | shares		5,115,380
Weighted-average price per share
Outstanding at the beginning of the period (in dollars per share) | $ / shares		$ 52.00
Granted (in dollars per share) | $ / shares		53.56
Exercised (in dollars per share) | $ / shares		34.71
Cancelled (in dollars per share) | $ / shares		78.27
Outstanding at the end of the period (in dollars per share) | $ / shares	$ 50.49	50.49
Vested at the end of the period (in dollars per share) | $ / shares	$ 41.81	$ 41.81
Total unrecognized compensation cost related to unvested stock options (in dollars) | $	$ 95.0	$ 95.0
Weighted average period over which unrecognized compensation costs are expected to be recognized	2 years 6 months 4 days