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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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