Benefit Plans - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Employee	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Matching contribution by the company to the plan	100.00%
Percentage of employees' eligible compensation	6.00%
Defined contribution plan, general and administrative expenses | $	$ 0.7	$ 0.7	$ 0.9
Plan name	401(K) plan
Defined Benefit Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of employees covered under defined benefit pension plan	28
Gain on reduction of pension liability | $				$ 2.2
Defined Benefit Pension Plan [Member] | Retired Employee [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of employees covered under defined benefit pension plan	2
Defined Benefit Pension Plan [Member] | Deferred Vested Termination [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of employees covered under defined benefit pension plan	4
Defined Benefit Pension Plan [Member] | Survivor [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of employees covered under defined benefit pension plan	1