Retirement Plans and Related Post Retirement Benefits - Summarizes the Components of the Net Periodic Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
SERP
Net Periodic Cost
Service cost — benefits earned during period	$ 194	$ 247	$ 295
Interest cost	843	721	624
Amortization of prior service cost	495	495	495
Amortization of (gains) losses	449	108	128
Net periodic cost	1,981	1,571	1,542
SERP Medical
Net Periodic Cost
Service cost — benefits earned during period	6	3	3
Interest cost	39	31	24
Amortization of prior service cost	26	25	25
Amortization of (gains) losses	26	0	0
Net periodic cost	$ 97	$ 59	$ 52