Financial Instruments - Summary of Bond Activity (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Fair value at beginning of period	$ 44,573	$ 43,142
Purchases	14,046	11,355
Redemptions	(940)	(2,900)
Maturities	(5,555)	(6,878)
Increase (decrease) in bond premium	67	(106)
Fair market value adjustment	267	(40)
Fair value at end of period	$ 52,458	$ 44,573