Financial Instruments - Summary of Investments (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Available For Sale Securities [Abstract]
Fair value at beginning of period	$ 60,659	$ 56,556
Purchases	3,447	27,216
Redemptions	(22,078)	(1,686)
Maturities	(10,889)	(22,272)
Decrease in bond premium	(750)	(460)
Fair market value adjustment	2	1,305
Fair value at end of period	$ 30,391	$ 60,659