Employee Stock Benefit Plans - Stock Options - Activity (Details) - Options to purchase common stock shares in Thousands	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Number of Shares
Outstanding at the beginning of the period (in shares) | shares	20,455
Granted (in shares) | shares	3,160
Exercised (in shares) | shares	(1,598)
Cancelled (in shares) | shares	(396)
Outstanding at the end of the period (in shares) | shares	21,621
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 11.92
Granted (in dollars per share) | $ / shares	16.63
Exercised (in dollars per share) | $ / shares	9.57
Cancelled (in dollars per share) | $ / shares	13.27
Outstanding at the end of the period (in dollars per share) | $ / shares	$ 12.75