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