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Restricted Stock, Performance Share and Cash Awards - Additional Information (Detail) |  |  |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Mar. 15, 2018 USD ($) shares | Mar. 16, 2017 USD ($) shares | Mar. 17, 2016 USD ($) shares | Mar. 31, 2018 shares | Mar. 31, 2017 shares | Mar. 31, 2016 shares | Dec. 31, 2018 USD ($) Times shares | Dec. 31, 2017 USD ($) shares | Dec. 31, 2016 USD ($) shares | Mar. 11, 2015 USD ($) shares
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |
Share-based compensation, shares outstanding | shares |  |  |  |  |  |  | 8,800,000 | 9,500,000 | 9,500,000 |
Shares granted in the period | shares |  |  |  |  |  |  | 439,100 | 476,350 | 479,167 |
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 |  |  |
Share-based compensation, shares outstanding | shares |  |  |  |  |  |  | 3,300,000 |  |  |
Shares granted in the period | shares |  |  |  |  |  |  | 3,300,000 |  |  |
Restricted Stock Units (RSUs) [Member] |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |
Shares granted in the period | shares |  |  |  | 420,200 | 477,500 | 466,600 | 439,100 | 476,350 | 479,167 |
Fair value of grants in period |  |  |  |  |  |  | $ 28,700,000 | $ 26,800,000 | $ 20,400,000 |
Share based payment award vesting date |  |  |  | Mar. 15, 2023 | Mar. 19, 2022 | Mar. 17, 2021 |  |  |  |
Restricted stock or unit expense |  |  |  |  |  |  | 27,200,000 | 19,600,000 | 18,200,000 |
Unvested Restricted Stock [Member] |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |
Total intrinsic value |  |  |  |  |  |  | 140,800,000 | 109,300,000 |  |
Equity awards with an aggregate fair value |  |  |  |  |  |  | 23,600,000 | 23,300,000 |  |
Performance Shares [Member] |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |
Equity awards with an aggregate fair value |  |  |  |  |  |  | $ 3,700,000 | 3,300,000 |  |
Shares authorized | shares | 78,200 | 86,250 | 72,900 |  |  |  |  |  |  |
Performance unit awards approved, Fair value | $ 5,300,000 | $ 4,900,000 | $ 3,200,000 |  |  |  |  |  |  |
Provisional awards, terms |  |  |  |  |  |  | Granted units for the 2018 and 2017 provisional awards will fully vest based on continuous employment through March 16, 2021 and 2020, respectively, and will be settled in shares of our common stock on a one-for-one basis as soon as practicable thereafter. |  |  |
Performance awards period, years |  |  |  |  |  |  | 1 year |  |  |
Vesting period, years |  |  |  |  |  |  | 2 years |  |  |
2018 Provisional Cash Awards [Member] |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |
Performance awards expiration date |  |  |  |  |  |  | Jan. 01, 2021 |  |  |
Provisional compensation cash award approved for future grant by compensation committee, units | shares |  |  |  |  |  |  | 190,000 |  |  |
Cash-based compensation awards, expenses |  |  |  |  |  |  | $ 0 |  |  |
2017 Performance Share Awards [Member] |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |
Provisional awards, terms |  |  |  |  |  |  | The 2018 and 2017 awards are subject to a three-year performance period that begins on January 1, 2018 and 2017, respectively, and vest on the three-year anniversary of the date of grant (March 15, 2021 and March 16, 2020) |  |  |
Performance awards period, years |  |  |  |  |  |  | 3 years |  |  |
Vesting period, years |  |  |  |  |  |  | 3 years |  |  |
Performance awards expiration date |  |  |  |  |  |  | Mar. 15, 2021 |  |  |
2017 Provisional Cash Awards [Member] |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |
Performance awards expiration date |  |  |  |  |  |  | Jan. 01, 2020 |  |  |
Cash-based compensation awards, expenses |  |  |  |  |  |  | $ 8,700,000 |  |  |
2016 Provisional Cash Awards [Member] |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |
Performance awards expiration date |  |  |  |  | Jan. 01, 2019 |  |  |  |  |
Cash-based compensation awards, expenses |  |  |  |  |  |  | $ 11,700,000 | 10,600,000 |  |
2015 Provisional Cash Awards [Member] |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |
Performance awards expiration date |  |  |  |  |  | Jan. 01, 2018 |  |  |  |
Provisional compensation cash award approved for future grant by compensation committee, units | shares |  |  |  |  |  |  | 269,000 |  |  |
Cash-based compensation awards, expenses |  |  |  |  |  |  | $ 15,800,000 | $ 9,300,000 | $ 6,600,000 |
2014 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 |  |  |  |  |  |  |  | 199,000 |  |
Grants vested in period fair value |  |  |  |  |  |  |  | $ 9,300,000 |  |
2013 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 |  |  |  |  |  |  |  |  | 246,000 |
Grants vested in period fair value |  |  |  |  |  |  |  |  | $ 11,200,000 |
Minimum [Member] | Restricted Stock Units (RSUs) [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 | 2 years |
Officer and Key Employees [Member] | Cash Awards [Member] |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |
Provisional awards, terms |  |  |  |  |  |  | He ultimate award value will be equal to the trailing twelve-month price of our common stock on December 31, 2020, 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. |  |  |
Performance awards period, years |  |  |  |  |  |  | 1 year |  |  |
Vesting period, years |  |  |  |  |  |  | 2 years |  |  |
Provisional compensation cash awards approved for future grant by compensation committee, value | $ 15,000,000 | $ 14,300,000 |  |  |  |  |  |  |  |
Provisional compensation cash award approved for future grant by compensation committee, units | shares | 219,000 | 255,000 |  | 242,000 |  |  |  |  |  |
Ultimate award value, multiples of original value of the units, minimum | Times |  |  |  |  |  |  | 0.5 |  |  |
Ultimate award value, multiples of original value of the units, maximum | Times |  |  |  |  |  |  | 1.5 |  |  |
Officer and Key Employees [Member] | 2016 Provisional Cash Awards [Member] |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |
Provisional compensation cash awards approved for future grant by compensation committee, value |  |  | $ 17,400,000 |  |  |  |  |  |  |
Provisional compensation cash award approved for future grant by compensation committee, units | shares |  |  | 397,000 |  | 385,000 |  |  |  |  |
Officer and Key Employees [Member] | 2015 Provisional Cash Awards [Member] |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |
Provisional compensation cash awards approved for future grant by compensation committee, value |  |  |  |  |  |  |  |  |  | $ 14,600,000
Provisional compensation cash award approved for future grant by compensation committee, units | shares |  |  |  |  |  | 294,000 |  |  |  | 315,000
Executive Officer [Member] | Minimum [Member] | Restricted Stock Units (RSUs) [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 |

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