Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,871	$ 3,411	$ 9,383	$ 10,842
Interest cost	7,149	6,348	21,241	18,953
Expected return on plan assets	(15,200)	(17,137)	(45,425)	(51,382)
Amortization of prior service cost (credit)	46	34	86	103
Amortization of actuarial losses (gains)	4,084	1,877	12,324	5,450
Net periodic benefit (credit) cost	(1,050)	(5,467)	(2,391)	(16,034)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	16	15	60	78
Interest cost	200	158	569	483
Amortization of prior service cost (credit)	(586)	561	(1,749)	(738)
Amortization of actuarial losses (gains)	(33)	(167)	(366)	(405)
Net periodic benefit (credit) cost	$ (403)	$ 567	$ (1,486)	$ (582)