Debt - Additional Information (Detail) - USD ($) shares in Thousands			3 Months Ended		9 Months Ended
	May 07, 2018	Apr. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Capitalized interest costs			$ 300,000	$ 200,000	$ 800,000	$ 900,000
Less unamortized debt issuance costs			17,989,000		$ 17,989,000		$ 20,477,000
Shares issued, shares					14,375
Shares issued					$ 217,312,000
Cash paid for debt prepayment		$ 31,600,000
Cash paid for debt issuance or prepayment costs					(20,703,000)	(38,130,000)
Gain (loss) on extinguishment of debt			(6,624,000)	$ 0	(19,562,000)	$ (39,639,000)
Senior Secured Revolving Facility
Debt Instrument [Line Items]
Principal available			100,000,000		100,000,000
Senior debt			69,401,000		69,401,000		$ 0
Senior Secured Revolving Facility | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Borrowings			143,000,000		$ 143,000,000
Debt maturity date					Oct. 19, 2020
Applicable margin					3.50%
Second Lien Credit Agreement
Debt Instrument [Line Items]
Cash paid for debt issuance or prepayment costs	$ (16,700,000)
Gain (loss) on extinguishment of debt	$ 12,900,000
Second Lien Credit Agreement | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Borrowings			200,000,000		$ 200,000,000
Less unamortized debt issuance costs			$ 2,600,000		$ 2,600,000
Debt maturity date					Apr. 19, 2023
Applicable margin					8.00%
Prepayment penalty percentage					2.00%
Cash paid for debt prepayment					$ 4,000,000
Second Lien Credit Agreement | London Interbank Offered Rate (LIBOR) | Minimum
Debt Instrument [Line Items]
Interest rate			1.00%		1.00%
Revolving Credit Facility
Debt Instrument [Line Items]
Less unamortized debt issuance costs			$ 3,800,000		$ 3,800,000