Financial Instruments - Summary of Bond Activity (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Fair value at beginning of period	$ 38,399	$ 30,011
Purchases	12,494	14,089
Sales		(245)
Maturities and calls	(8,865)	(6,426)
Increase in bond premium	148	1,155
Fair market value adjustment	39	(185)
Fair value at end of period	$ 42,215	$ 38,399