Postretirement Benefits - Activity for the SERP (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Retirement Benefits [Abstract]
Premiums paid for insurance coverage	$ 2,394	$ 2,997
Company owned life insurance	47,406	44,285
Exchange traded funds	62,944	52,659
Total investments of SERP	$ 110,350	$ 96,944