Postretirement Benefit Plans - Defined Contribution Plan Expenses for the Company's Retirement Savings Plans and Profit Sharing Plan (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution expense	$ 12,000	$ 11,132	$ 9,616
Retirement Contributions [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution expense	4,998	4,902	4,644
Profit Sharing Contributions [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution expense	$ 7,002	$ 6,230	$ 4,972