Postretirement Benefits - Activity for the SERP (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Retirement Benefits [Abstract]
Premiums paid for COLI	$ 2,394	$ 2,997	$ 2,050
Total investments:
COLI	47,733	44,285
Exchange traded funds	65,585	52,659
Total investments of SERP	$ 113,318	$ 96,944