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Stock Option Plans - Additional information (Detail) - USD ($) $ / shares in Units, $ in Millions |  |  | 3 Months Ended | 9 Months Ended
--- | --- | --- | --- | ---
Mar. 12, 2020 | Mar. 14, 2019 | Sep. 30, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Stock Option Plans [Line Items] |  |  |  |  |  |  |  |  |  |
Compensation expense related to stock option grants |  |  | $ 3.4 | $ 3.3 | $ 3.4 | $ 3.4 | $ 3.5 | $ 3.6 | $ 10.1 | $ 10.5
Total intrinsic value of options exercised |  |  |  |  |  |  |  |  | 68.7 | $ 60.9
Total unrecognized compensation cost related to nonvested options |  |  | $ 32.1 |  |  |  |  |  | $ 32.1 |
Weighted average period, years |  |  |  |  |  |  |  |  | 4 years |
Black-Scholes Option Pricing Model [Member] |  |  |  |  |  |  |  |  |  |
Stock Option Plans [Line Items] |  |  |  |  |  |  |  |  |  |
Weighted average fair value per option for all options |  |  |  |  |  |  |  |  | $ 9.99 | $ 10.71
Long Term Incentive Plan [Member] |  |  |  |  |  |  |  |  |  |
Stock Option Plans [Line Items] |  |  |  |  |  |  |  |  |  |
Maximum number of shares available |  |  |  |  |  |  |  |  | 2,200,000 |
Shares available for grant |  |  | 11,400,000 |  |  |  |  |  | 11,400,000 |
Long Term Incentive Plan [Member] | Officer and Key Employees [Member] |  |  |  |  |  |  |  |  |  |
Stock Option Plans [Line Items] |  |  |  |  |  |  |  |  |  |
Shares available for grant | 1,590,740 | 1,283,300 |  |  |  |  |  |  |  |
Stock options granted, exercise percentage, on the third anniversary date of the grant | 34.00% | 34.00% |  |  |  |  |  |  |  |
Stock options granted, exercise percentage, on the fourth anniversary date of the grant | 33.00% | 33.00% |  |  |  |  |  |  |  |
Stock options granted, exercise percentage, on the fifth anniversary date of the grant | 33.00% | 33.00% |  |  |  |  |  |  |  |
Long Term Incentive Plan [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |
Stock Option Plans [Line Items] |  |  |  |  |  |  |  |  |  |
Minimum exercise price of stock options, percent of fair market value of a share of common stock on the date of grant |  |  |  |  |  |  |  |  | 100.00% |
Period of service from grant date stock options awarded not subject to forfeiture |  |  |  |  |  |  |  |  | 2 years | 2 years
Long Term Incentive Plan [Member] | Minimum [Member] | Executive Officer [Member] |  |  |  |  |  |  |  |  |  |
Stock Option Plans [Line Items] |  |  |  |  |  |  |  |  |  |
Minimum age of employee with not subject to award forfeiture on condition compliance |  |  |  |  |  |  |  |  | 55 years | 55 years
Long Term Incentive Plan [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |
Stock Option Plans [Line Items] |  |  |  |  |  |  |  |  |  |
Maximum period for the exercise of stock options, years |  |  |  |  |  |  |  |  | 7 years | 7 years
2017 Long Term Incentive Plan [Member] |  |  |  |  |  |  |  |  |  |
Stock Option Plans [Line Items] |  |  |  |  |  |  |  |  |  |
Number Of Periods Options Expire |  |  |  |  |  |  |  |  | 7 years |

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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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