Stock-Based Compensation Expense - Summary of Restricted Stock and Restricted Stock Unit Activity (Detail)	6 Months Ended
	Jun. 30, 2019 $ / shares shares
Restricted Stock [Member]
Shares
Unvested balance, Beginning balance (in shares) | shares	21,677
Granted (in shares) | shares	0
Vested (in shares) | shares	(21,677)
Forfeited (in shares) | shares	0
Unvested balance, Ending balance (in shares) | shares	0
Weighted-Average Grant Date Fair Value
Beginning balance (in dollars per share) | $ / shares	$ 10.88
Granted (in dollars per share) | $ / shares	0
Vested (in dollars per share) | $ / shares	10.88
Forfeited (in dollars per share) | $ / shares	0
Ending Balance (in dollars per share) | $ / shares	$ 0
Restricted Stock Units (RSUs) [Member]
Shares
Unvested balance, Beginning balance (in shares) | shares	2,773,773
Granted (in shares) | shares	1,509,033
Vested (in shares) | shares	(594,764)
Forfeited (in shares) | shares	(174,261)
Unvested balance, Ending balance (in shares) | shares	3,513,781
Weighted-Average Grant Date Fair Value
Beginning balance (in dollars per share) | $ / shares	$ 21.21
Granted (in dollars per share) | $ / shares	41.83
Vested (in dollars per share) | $ / shares	22.88
Forfeited (in dollars per share) | $ / shares	25.31
Ending Balance (in dollars per share) | $ / shares	$ 29.58