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Stock-Based Compensation - Narrative (Detail) - USD ($)			3 Months Ended		9 Months Ended		12 Months Ended
	Apr. 05, 2019	Apr. 11, 2012	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2012	May 09, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation cost			$ 21,150,000	$ 19,180,000	$ 57,554,000	$ 54,835,000
Options granted, contractual period, years					10 years
Options granted (in shares)					1,831,180	431,924
Unrecognized stock-based compensation cost for options			26,843,000		$ 26,843,000
Restrcited Stock Unit vesting right (in shares)					1
Restrcited Stock Unit vesting right, fair value (in shares)					1
Performance Units vesting right (in shares)					1
Performance Units vesting right, fair value (in shares)					1
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award, vesting period					3 years
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award, vesting period					5 years
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average remaining requisite vesting period					3 years 3 months 18 days
Restricted Share Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average remaining requisite vesting period					2 years 4 months 24 days
Shares to be issued on vested awards other than options (in shares)							34,092
Unrecognized stock-based compensation cost			21,417,000		$ 21,417,000
Restricted Share Units (RSUs) | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award, vesting period					3 months
Restricted Share Units (RSUs) | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award, vesting period					4 years
Performance Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average remaining requisite vesting period					1 year 10 months 24 days
Unrecognized stock-based compensation cost			$ 65,843,000		$ 65,843,000
Performance Units | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award, vesting period					6 months
Performance Units | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award, vesting period					3 years
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value per share allowed to eligible employees to purchase through payroll deductions (in percentage)					90.00%
Maximum percentage of employee's base salary allowed to be purchased (in percentage)			15.00%		15.00%
Maximum dollar amount of common shares allowed to be purchased					$ 25,000
Common shares reserved for issuance (in shares)			4,200,000		4,200,000
Issuance of common shares under the employee stock purchase plan (in shares)					216,378	241,953
Compensation expense for ESPP			$ 335,000	$ 307,000	$ 1,050,000	$ 983,000
2007 Omnibus Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amended Omnibus Plan, increase in number of common shares authorized for issuance (in shares)	8,000,000	5,593,200						8,858,823
Number of common shares authorized for issuance (in shares)	23,000,000	15,000,000
2017 Omnibus Incentive Compensation Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common shares authorized for issuance (in shares)									15,000,000

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