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Stock award plan - (Tables)

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Summary of stock option activity

A summary of stock option activity is as follows:

​

​	​	​	​	​	​	​	​	​	​	​	​
​		​		​	​		Weighted-			​	​
​	​	​	​	Weighted-		​	average	​	​	Aggregate
​	​	​	​	average		​	remaining	​	​	intrinsic
​	​	Number of	​	exercise		​	contractual	​	​	value(in
​	​	options	​	price		​	term	​	​	thousands)
​	​	​	​	​	​	​	​		​	​	​
Outstanding at December 31, 2017		6,448,642	​	$	29.00				​	​
Granted		3,181,623	​	$	26.64				​	​
Exercised		(633,973)	​	$	18.61				​	​
Forfeited		(461,934)	​	$	35.36				​	​
Outstanding at December 31, 2018		8,534,358	​	$	28.58				​	​
Granted		3,977,995	​	$	35.81				​	​
Exercised		(949,887)	​	$	19.25				​	​
Forfeited		(518,527)	​	$	35.27				​	​
Outstanding at December 31, 2019		11,043,939	​	$	31.67				​	​
Granted		2,777,975	​	$	51.06				​	​
Exercised		(3,268,452)	​	$	24.25				​	​
Forfeited/Cancelled		(889,785)	​	$	42.14				​	​
Outstanding at December 31, 2020		9,663,677	​	$	38.72		7.37	years	​	$	218,374
Vested or Expected to vest at December 31, 2020		4,910,488	​	$	41.24		8.49	years	​	$	97,320
Exercisable at December 31, 2020		4,249,063	​	$	35.24		5.92	years	​	$	112,228

​

Schedule of assumptions used to estimate fair values of grants made on the date of grant

The fair values of grants made in the years ended December 31, 2020, 2019 and 2018 were contemporaneously estimated on the date of grant using the following assumptions:

​

​	​	​	​	​	​	​
​		​	​	​	​	​
​		2020		2019		2018
Risk-free interest rate		0.34% - 1.45%		1.58% - 2.63%		2.25% - 3.10%
Expected volatility		87% - 89%		62% - 92%		64% - 90%
Expected term		5.75 years		5.75 - 6.11 years		5.03 - 10.00 years

​

Summary of information on the Company's restricted stock

The following table summarizes information on the Company’s restricted stock awards and units:

​

​	​	​	​	​	​
​	​	Restricted Stock Awards and Units
​	​	​	​	Weighted
​	​	​	​	Average
​	​	​	​	Grant
​	​	Number of	​	Date
​		Shares		Fair Value
Unvested at December 31, 2019	​	642,419	​	$	24.50
Granted		663,045	​	$	51.32
Vested		(232,975)	​	$	23.87
Forfeited		(90,431)	​	$	36.44
Unvested at December 31, 2020		982,058	​	$	41.78

​

Schedule of share-based compensation expense recorded in the statement of operations

The Company recorded share-based compensation expense in the statement of operations related to incentive stock options, nonstatutory stock options, restricted stock awards, restricted stock units and the ESPP as follows:

​

​	​	​	​	​	​	​	​	​	​
​	​	Year ended December 31,
​	​	2020			2019			2018
Research and development	​	$	38,716	​	$	20,836	​	$	16,096
Selling, general and administrative	​		31,609	​		21,298	​		17,156
Total	​	$	70,325	​	$	42,134	​	$	33,252

​

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Tabular disclosure of the changes in outstanding nonvested restricted stock shares.

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Tabular disclosure of allocation of amount expensed and capitalized for award under share-based payment arrangement to statement of income or comprehensive income and statement of financial position. Includes, but is not limited to, corresponding line item in financial statement.

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Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value.

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Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions.

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