Financial Instruments - Summary of Investments (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Available For Sale Securities [Abstract]
Fair value at beginning of period	$ 56,556	$ 51,101
Purchases	13,920	18,426
Redemptions	(311)	(1,793)
Maturities	(10,245)	(10,445)
Decrease in bond premium	(357)	(377)
Fair market value adjustment	1,144	(356)
Fair value at end of period	$ 60,707	$ 56,556