Postretirement Benefit Plans - Defined Contribution Plan Expenses for the Company's Retirement Savings Plans and Profit Sharing (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution expense	$ 2,497	$ 2,583	$ 7,970	$ 9,017
Retirement Savings Plans [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution expense	1,738	1,275	5,201	3,758
Profit Sharing Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution expense	$ 759	$ 1,308	$ 2,769	$ 5,259