Financial Instruments - Summary of Bond Activity (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Fair value at beginning of period	$ 44,573	$ 43,142
Purchases	15,546	11,355
Sales	(940)	(2,900)
Maturities and calls	(7,845)	(6,878)
Decrease in bond premium	(160)	(106)
Fair market value adjustment	(73)	(40)
Fair value at end of period	$ 51,101	$ 44,573