Financial Instruments - Summary of Bond Activity (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Fair value at beginning of period	$ 51,101	$ 44,573
Purchases	10,745	15,546
Redemptions	(1,331)	(940)
Maturities	(7,370)	(7,845)
Decrease in bond premium	(219)	(160)
Fair market value adjustment	(538)	(73)
Fair value at end of period	$ 52,388	$ 51,101