Reconciliation of Beginning and Ending Balances of Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - Fair Value Measurements, Level 3 - Fair Value Measurements, Recurring $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 22,469
Change in fair value	1,021
Ending Balance	$ 23,490