Postretirement Benefits - Activity for the SERP (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Retirement Benefits [Abstract]
Premiums paid for COLI	$ 2,480	$ 2,394
Total investments:
COLI	50,985		$ 47,733
Exchange traded funds	68,862		65,585
Total investments of SERP	$ 119,847		$ 113,318