Postretirement Benefit Plans - Defined Contribution Plan Expenses for Company's Qualified Contribution Plans (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution plan expenses	$ 3,843	$ 3,433	$ 7,748	$ 6,166
Retirement Savings Contributions [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution plan expenses	1,978	1,950	4,017	3,708
Profit Sharing Contributions Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution plan expenses	$ 1,865	$ 1,483	$ 3,731	$ 2,458