Postretirement Benefits - Activity for the SERP (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Retirement Benefits [Abstract]
Premiums paid for insurance coverage	$ 444	$ 2,997
Company owned life insurance	45,326	44,285
Exchange traded funds	61,048	52,659
Total investments of SERP	$ 106,374	$ 96,944