Postretirement Benefits - Activity for the SERP (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Retirement Benefits [Abstract]
Premiums paid for COLI	$ 443	$ 444
Total investments:
COLI	48,497		$ 47,733
Exchange traded funds	55,303		65,585
Total investments of SERP	$ 103,800		$ 113,318