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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution plan expenses	$ 2,179	$ 2,774	$ 5,470	$ 5,473
Retirement Savings Contributions [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution plan expenses	1,215	1,706	3,624	3,463
Profit Sharing Contributions [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution plan expenses	$ 964	$ 1,068	$ 1,846	$ 2,010