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Restricted Stock, Performance Share and Cash Awards - Additional Information (Detail) |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 16, 2017 USD ($) shares | Mar. 17, 2016 USD ($) shares | Sep. 30, 2017 USD ($) shares | Mar. 31, 2017 USD ($) shares | Sep. 30, 2016 USD ($) shares | Mar. 31, 2016 USD ($) shares | Sep. 30, 2017 USD ($) Times shares | Sep. 30, 2016 USD ($) shares | Dec. 31, 2016 USD ($) shares | Mar. 31, 2015 shares | Mar. 11, 2015 USD ($) shares | Mar. 12, 2014 USD ($) shares
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share-based compensation, shares outstanding | shares |  |  | 10,700,000 |  |  |  | 10,700,000 |  | 10,300,000 |  |  |
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 |  |  |  | $ 4,000,000 |  |  |  |  |
Share-based compensation, shares outstanding | shares |  |  | 4,000,000 |  |  |  | 4,000,000 |  |  |  |  |
Shares granted in the period | shares |  |  |  |  |  |  | 4,000,000 |  |  |  |  |
Restricted Stock Units (RSUs) [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Shares granted in the period | shares |  |  |  | 477,500 |  | 466,600 |  |  |  |  |  |
Fair value of grants in period |  |  |  | $ 26,800,000 |  | $ 20,400,000 |  |  |  |  |  |
Share based payment award vesting date |  |  |  | Mar. 16, 2021 |  | Mar. 11, 2020 |  |  |  |  |  |
Restricted stock or unit expense |  |  | $ 4,700,000 |  | $ 4,200,000 |  | $ 15,200,000 | $ 14,200,000 |  |  |  |
Restricted Stock Units (RSUs) [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 |  |  |  |  |  |
Restricted Stock Units (RSUs) [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 |  |  |  |  |  |
Unvested Restricted Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total intrinsic value |  |  | 107,600,000 |  | 79,700,000 |  | 107,600,000 | 79,700,000 |  |  |  |
Equity awards with an aggregate fair value |  |  |  |  |  |  | $ 23,300,000 | 14,200,000 |  |  |  |
Performance Shares [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Shares authorized | shares | 86,250 | 72,900 |  |  |  |  |  |  |  |  |  |
Performance unit awards approved, Fair value | $ 4,900,000 | $ 3,200,000 |  |  |  |  |  |  |  |  |  |
Provisional awards, terms |  |  |  |  |  |  | Granted units for the 2017 provisional awards will fully vest based on continuous employment through March 16, 2020 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 |  |  |  |  |
2017 Performance Share Awards [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Provisional awards, terms |  |  |  |  |  |  | The 2017 awards are subject to a three-year performance period that begins on January 1, 2017, and vest on the three-year anniversary of the date of grant (March 16, 2020). |  |  |  |  |
Performance awards period, years |  |  |  |  |  |  | 3 years |  |  |  |  |
Vesting period, years |  |  |  |  |  |  | 3 years |  |  |  |  |
Performance awards expiration date |  |  |  |  |  |  | Mar. 16, 2020 |  |  |  |  |
Cash Awards [Member] | Officer and Key Employees [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, 2019, 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 | $ 14,300,000 | $ 17,400,000 |  |  |  |  |  |  |  |  | $ 14,600,000 | $ 10,800,000
Provisional compensation cash award approved for future grant by compensation committee, units | shares | 255,000 | 397,000 |  | 383,000 |  | 294,000 |  |  |  | 220,000 | 315,000 | 229,000
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 |  |  |  |  |
2017 Provisional Cash Awards [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cash-based compensation awards, expenses |  |  |  |  |  |  | $ 0 |  |  |  |  |
2016 Provisional Cash Awards [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cash-based compensation awards, expenses |  |  | 2,600,000 |  |  |  | 7,900,000 |  | $ 0.0 |  |  |
2015 Provisional Cash Awards [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cash-based compensation awards, expenses |  |  | $ 2,000,000 |  | 1,700,000 |  | $ 7,000,000 | 4,900,000 |  |  |  |
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 |  |  |  | 199,000 |  |  |  |  |
Cash-based compensation awards, expenses |  |  |  |  | $ 1,200,000 |  |  | $ 3,100,000 |  |  |  |
Grants vested in period fair value |  |  | $ 9,300,000 |  |  |  | $ 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 |  |  |  |  | 245,500 |  |  | 245,500 |  |  |  |
Grants vested in period fair value |  |  |  |  | $ 11,200,000 |  |  | $ 11,200,000 |  |  |  |

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