Debt - Additional Information (Detail) - USD ($)		9 Months Ended
	Jul. 14, 2017	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jul. 13, 2017
Line of Credit Facility [Line Items]
Revolving credit borrowing		$ 163,600,000	$ 99,200,000	$ 99,600,000
Letters of credit outstanding		3,300,000
Capital lease and other obligations		4,700,000	$ 3,800,000	$ 3,900,000
Minimum [Member]
Line of Credit Facility [Line Items]
Fall of excess committed borrowing availability		17,500,000
Maximum [Member]
Line of Credit Facility [Line Items]
Fall of excess committed borrowing availability		31,300,000
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, expiration date	Jul. 14, 2022
Amortization period on real estate component of borrowing base	12 years 6 months
Revolving credit borrowing		$ 163,600,000
Weighted average interest rate		4.10%
Excess committed borrowing availability		$ 35,800,000
Unused commitment fees		0.25%
Fixed charge coverage ratio of credit facility		100.00%
Revolving Credit Facility [Member] | Asset Backed Securities [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 250,000,000				$ 160,000,000
Line of credit facility, conditional maximum borrowing capacity	300,000,000
Uncommitted conditional increased borrowing capacity during period	$ 50,000,000