Financial Instruments - Summary of Bond Activity (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Fair value at beginning of period	$ 43,142	$ 38,399
Purchases	7,300	15,429
Redemptions	(2,150)	(987)
Maturities	(4,982)	(9,677)
Change in bond premium	(299)	172
Fair market value adjustment	416	(194)
Fair value at end of period	$ 43,427	$ 43,142