Summary of Fair Value Transfers Between Level I and Level II (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers from Level I into Level II	[1]	$ 0	$ 0
Transfers from Level II into Level I	[2]	$ 447

[1] Transfers out of Level I represent those financial instruments for which restrictions exist and adjustments were made to an otherwise observable price to reflect fair value at the reporting date.

[2] Transfers into Level I represent those financial instruments for which an unadjusted quoted price in an active market became available for the identical asset.