Postretirement Benefit Plans - Defined Contribution Plan Expenses for Company's Qualified Contribution Plans (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution plan expenses	$ 2,733	$ 3,291
Retirement Savings Contributions [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution plan expenses	1,758	2,409
Profit Sharing Contributions [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution plan expenses	$ 975	$ 882