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Stock Option Plans - Additional information (Detail) - USD ($) $ / shares in Units, $ in Millions |  |  | 3 Months Ended | 6 Months Ended
--- | --- | --- | --- | ---
Mar. 12, 2020 | Mar. 14, 2019 | Jun. 30, 2020 | Mar. 31, 2020 | Jun. 30, 2019 | Mar. 31, 2019 | Jun. 30, 2020 | Jun. 30, 2019
Stock Option Plans [Line Items] |  |  |  |  |  |  |  |
Compensation expense related to stock option grants |  |  | $ 3.3 | $ 3.4 | $ 3.5 | $ 3.6 | $ 6.7 | $ 7.1
Total intrinsic value of options exercised |  |  |  |  |  |  | 47.1 | $ 48.3
Total unrecognized compensation cost related to nonvested options |  |  | $ 36.0 |  |  |  | $ 36.0 |
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 |  |  | 11,300,000 |  |  |  | 11,300,000 |
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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