Postretirement Benefit Plans - Defined Contribution Plan Expenses for Company's Qualified Contribution Plans (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution plan expenses	$ 3,291	$ 2,699
Retirement Savings Contributions [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution plan expenses	2,409	1,757
Profit Sharing Contributions [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution plan expenses	$ 882	$ 942