Summary of Required Compliance Ratios in Connection with Terms of Credit Agreement (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Covenant, Tangible net worth	$ 136,170
Tangible net worth	$ 190,002
Total leverage	41.20%
Secured leverage	11.70%
Unencumbered leverage	37.70%
Fixed charge coverage	3.80%
Maximum
Debt Instrument [Line Items]
Covenant, Total leverage	60.00%
Covenant, Secured leverage	30.00%
Covenant, Unencumbered leverage	60.00%
Minimum
Debt Instrument [Line Items]
Covenant, Fixed charge coverage	1.50%