Postretirement Benefit Plans - Defined Contribution Plan Expenses for the Company's Retirement Savings Plans and Profit Sharing (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution expense	$ 2,774	$ 3,332	$ 5,473	$ 6,434
Retirement Savings Plans [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution expense	1,706	1,224	3,463	2,483
Profit Sharing [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution expense	$ 1,068	$ 2,108	$ 2,010	$ 3,951