Summary of Long-Term Debt (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Credit facility 2	[1]	$ 77,000		$ 25,000
Second Lien Facility		200,000		0
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums	[2]	11,733		0
Long-term debt		265,267		25,000
Debt Instrument, Unamortized Discount	[2]	(3,839)		0
Unamortized Debt Issuance Expense	[2]	(7,894)		0
Long-term Debt		277,000		25,000
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net		$ 265,267		$ 25,000
Second Lien Facility [Member]
Debt Instrument [Line Items]
Credit facility interest rate option one, applicable margin rate over Adjusted LIBOR		7.00%
Second Lien Facility, Interest Rate			8.57%

[1] 2 Issuance costs of the Credit Facility, which represent costs attributable to the access to credit over its contractual term, have been presented as a component of Other assets (see Note 13) and are being amortized over the term of the Credit Facility using the straight-line method.

[2] 1 Discount and issuance costs of the Second Lien Facility are being amortized over the term of the underlying loan using the effective-interest method.