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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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