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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution plan expenses	$ 3,433	$ 2,179	$ 6,166	$ 5,470
Retirement Savings Contributions [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution plan expenses	1,950	1,215	3,708	3,624
Profit Sharing Contributions Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution plan expenses	$ 1,483	$ 964	$ 2,458	$ 1,846