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Stock Option Plans - Additional information (Detail) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended						9 Months Ended
	Mar. 16, 2021	Mar. 12, 2020	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Stock Option Plans [Line Items]
Compensation expense related to stock option grants			$ 4.7	$ 4.5	$ 3.7	$ 3.4	$ 3.3	$ 3.4	$ 12.9	$ 10.1
Total intrinsic value of options exercised									93.6	$ 68.7
Total unrecognized compensation cost related to nonvested options			$ 52.4						$ 52.4
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									$ 23.38	$ 9.99
Long Term Incentive Plan [Member]
Stock Option Plans [Line Items]
Maximum number of shares available									1,800,000
Shares available for grant			9,500,000						9,500,000
Long Term Incentive Plan [Member] | Officer and Key Employees [Member]
Stock Option Plans [Line Items]
Shares available for grant	1,640,000	1,590,740
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] | Maximum [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%
Maximum period for the exercise of stock options, years									7 years	7 years
Long Term Incentive Plan [Member] | Minimum [Member]
Stock Option Plans [Line Items]
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
2017 Long Term Incentive Plan [Member]
Stock Option Plans [Line Items]
Number Of Periods Options Expire									7 years

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