Employee Stock Benefit Plans - Stock Options - Activity (Details) - Options to purchase common stock shares in Thousands	6 Months Ended
	Jun. 30, 2018 $ / shares shares
Number of Shares
Outstanding at the beginning of the period (in shares) | shares	21,086
Granted (in shares) | shares	3,041
Exercised (in shares) | shares	(1,607)
Cancelled (in shares) | shares	(464)
Outstanding at the end of the period (in shares) | shares	22,056
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 12.90
Granted (in dollars per share) | $ / shares	14.74
Exercised (in dollars per share) | $ / shares	10.42
Cancelled (in dollars per share) | $ / shares	14.79
Outstanding at the end of the period (in dollars per share) | $ / shares	$ 13.30