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Restricted Stock, Performance Share and Cash Awards - Additional Information (Detail)			3 Months Ended				9 Months Ended
	Mar. 12, 2020 USD ($) shares	Mar. 14, 2019 USD ($) shares	Sep. 30, 2020 USD ($) shares	Mar. 31, 2020 USD ($) shares	Sep. 30, 2019 USD ($) shares	Mar. 31, 2019 USD ($) shares	Sep. 30, 2020 USD ($) TIME shares	Sep. 30, 2019 USD ($) shares	Dec. 31, 2019 shares	Mar. 31, 2018 shares	Mar. 15, 2018 USD ($) shares	Mar. 16, 2017 USD ($) shares
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Share-based compensation, shares outstanding | shares			8,000,000.0				8,000,000.0		7,900,000
Restricted Stock Units (RSUs) [Member]
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Shares granted in the period | shares				405,900		399,900
Fair value of grants in period				$ 34,900,000		$ 31,800,000
Share based payment award vesting date				Mar. 12, 2025		Mar. 14, 2024
Period of service from grant date stock options awarded not subject to forfeiture							2 years	2 years
Restricted stock or unit expense			$ 8,900,000		$ 7,700,000		$ 29,000,000.0	$ 22,500,000
Unvested Restricted Stock [Member]
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Total intrinsic value			246,400,000		201,900,000		246,400,000	201,900,000
Equity awards with an aggregate fair value							30,600,000	2,000,000.0
Performance Shares [Member]
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Equity awards with an aggregate fair value							$ 12,500,000	5,700,000
Shares authorized | shares	82,500	73,600
Performance share awards approved, Fair value	$ 7,100,000	$ 5,800,000
Performance awards period, years	1 year
Vesting period, years	2 years
2017 Performance Share Awards [Member]
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Provisional awards, terms							The 2020 and 2019 awards are subject to a three-year performance period that began on January 1, 2020 and 2019, respectively, and vest on the three-year anniversary of the date of grant (March 12, 2023 and March 14, 2022).
Cash-based compensation awards, expenses					2,700,000			7,700,000
Performance awards expiration date							Jan. 01, 2020
2019 Provisional Cash Awards [Member]
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Cash-based compensation awards, expenses					0			0
Performance awards expiration date							Jan. 01, 2022
2020 Provisional Cash Awards [Member]
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Cash-based compensation awards, expenses			2,900,000		2,900,000		$ 7,200,000	7,200,000
2018 Provisional Cash Awards [Member]
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Cash-based compensation awards, expenses			$ 2,700,000		$ 2,500,000
Performance awards expiration date						Jan. 01, 2021
Restricted Stock Unit Provisional Cash Awards [Member]
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Cash-based compensation awards, expenses							$ 6,800,000	$ 6,600,000
2017 Provisional Cash Awards [Member]
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Provisional compensation cash award approved for future grant by compensation committee, units | shares			221,600				221,600
Cash-based compensation awards, expenses							$ 18,900,000
2016 Provisional Cash Awards [Member]
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Provisional compensation cash award approved for future grant by compensation committee, units | shares					341,000			341,000
Cash-based compensation awards, expenses								$ 22,400,000
Executive Officer [Member] | Restricted Stock Units (RSUs) [Member] | Minimum [Member]
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Minimum age of employee with not subject to award forfeiture on condition compliance	55 years						55 years	55 years
Officer and Key Employees [Member]
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Performance awards period, years	1 year
Officer and Key Employees [Member] | Cash Awards [Member]
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Provisional awards, terms							The ultimate award value will be equal to the trailing twelve-month price of our common stock on December 31, 2022, multiplied by the number of units subject to the award, but limited to between 0.5 and 1.5 times the original value of the units determined as of the grant date.
Vesting period, years	3 years
Provisional compensation cash awards approved for future grant by compensation committee, value	$ 18,400,000	$ 16,500,000									$ 15,000,000.0	$ 14,300,000
Provisional compensation cash award approved for future grant by compensation committee, units | shares	213,000	206,800		200,000		190,000				242,000	219,000	255,000
Ultimate award value, multiples of original value of the units, minimum | TIME							0.5
Ultimate award value, multiples of original value of the units, maximum | TIME							1.5
Cash-based compensation awards, expenses							$ 0
Long Term Incentive Plan [Member]
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Share-based compensation, shares outstanding, Value			$ 4,000,000.0				$ 4,000,000.0
Share-based compensation, shares outstanding | shares			2,200,000				2,200,000
Shares granted in the period | shares							2,200,000
Long Term Incentive Plan [Member] | Minimum [Member]
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Period of service from grant date stock options awarded not subject to forfeiture							2 years	2 years
Long Term Incentive Plan [Member] | Executive Officer [Member] | Minimum [Member]
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Minimum age of employee with not subject to award forfeiture on condition compliance							55 years	55 years

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