Postretirement Benefits - Activity for the SERP (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Retirement Benefits [Abstract]
Premiums paid for COLI	$ 443	$ 444
Total investments:
COLI	48,855		$ 47,733
Exchange traded funds	66,254		65,585
Total investments of SERP	$ 115,109		$ 113,318