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Equity-Based Compensation - Summarizes Information Related to Awards (Detail) - $ / shares	1 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted stock awards
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Non-vested shares outstanding at December 31, beginning balance		3,741,898
Granted, shares		3,546,280
Vested, shares		(1,826,855)
Forfeited, shares		(324,767)
Non-vested shares outstanding at December 31, ending balance		5,136,556	3,741,898
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Non-vested shares outstanding at December 31, beginning balance, weighted average grant date fair value per share (in dollars per share)		$ 10.78
Granted, weighted average grant date fair value per share (in dollars per share)		10.20	$ 15.03	$ 5.63
Vested, weighted average grant date fair value per share (in dollars per share)		9.71
Forfeited, weighted average grant date fair value per share (in dollars per share)		10.03
Non-vested shares outstanding at December 31, ending balance, weighted average grant date fair value per share (in dollars per share)		$ 10.81	$ 10.78
Performance share unit awards
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Non-vested shares outstanding at December 31, beginning balance		1,872,710
Granted, shares		854,400
Vested, shares		(564,068)	0
Forfeited, shares		(130,830)
Non-vested shares outstanding at December 31, ending balance		2,032,212	1,872,710
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Non-vested shares outstanding at December 31, beginning balance, weighted average grant date fair value per share (in dollars per share)		$ 11.10
Granted, weighted average grant date fair value per share (in dollars per share)		12.71	$ 16.89	$ 3.00
Vested, weighted average grant date fair value per share (in dollars per share)		7.01
Forfeited, weighted average grant date fair value per share (in dollars per share)		41.71
Non-vested shares outstanding at December 31, ending balance, weighted average grant date fair value per share (in dollars per share)		$ 10.94	$ 11.10
Phantom units
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Non-vested shares outstanding at December 31, beginning balance		99,100
Granted, shares		87,480
Vested, shares		(29,254)
Forfeited, shares		(14,237)
Non-vested shares outstanding at December 31, ending balance		143,089	99,100
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Non-vested shares outstanding at December 31, beginning balance, weighted average grant date fair value per share (in dollars per share)		$ 16.40
Granted, weighted average grant date fair value per share (in dollars per share)		23.91
Forfeited, weighted average grant date fair value per share (in dollars per share)		17.86
Non-vested shares outstanding at December 31, ending balance, weighted average grant date fair value per share (in dollars per share)		$ 20.85	$ 16.40
Restricted unit awards
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Non-vested shares outstanding at December 31, beginning balance		11,766
Granted, shares	17,260
Vested, shares		(11,766)
Forfeited, shares		0
Non-vested shares outstanding at December 31, ending balance		17,260	11,766
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Non-vested shares outstanding at December 31, beginning balance, weighted average grant date fair value per share (in dollars per share)		$ 17.00
Granted, weighted average grant date fair value per share (in dollars per share)		17.55
Vested, weighted average grant date fair value per share (in dollars per share)		17.00
Forfeited, weighted average grant date fair value per share (in dollars per share)		0.00
Non-vested shares outstanding at December 31, ending balance, weighted average grant date fair value per share (in dollars per share)		$ 17.55	$ 17.00

X

- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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