Financial Instruments - Summary of Bond Activity (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Fair value at beginning of period	$ 43,142	$ 38,399
Purchases	11,355	15,429
Sales	(2,900)	(987)
Maturities and calls	(6,878)	(9,677)
Decrease (increase) in bond premium	(106)	172
Fair market value adjustment	(40)	(194)
Fair value at end of period	$ 44,573	$ 43,142