Postretirement Benefit Plans - Defined Contribution Plan Expenses for Company's Qualified Contribution Plans (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution plan expenses	$ 3,431	$ 3,446	$ 11,179	$ 9,611
Retirement Savings Contributions [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution plan expenses	2,055	1,813	6,072	5,521
Profit Sharing Contributions Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution plan expenses	$ 1,376	$ 1,633	$ 5,107	$ 4,090