Postretirement Benefit Plans - Defined Contribution Plan Expenses for the Company's Retirement Savings Plans and Profit Sharing Plan (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution expense	$ 2,583	$ 2,899	$ 9,017	$ 8,947
Retirement Savings Plans [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution expense	1,275	1,304	3,758	3,849
Profit Sharing Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Total defined contribution expense	$ 1,308	$ 1,595	$ 5,259	$ 5,098