Financial Instruments - Summary of Bond Activity (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Fair value at beginning of period	$ 44,573	$ 43,142
Purchases	10,418	11,355
Redemptions	(940)	(2,900)
Maturities and calls	(2,485)	(6,878)
Change in bond premium	359	(106)
Fair market value adjustment	148	(40)
Fair value at end of period	$ 52,073	$ 44,573