Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,486	$ 3,258
Interest cost	5,710	7,049
Expected return on plan assets	(17,993)	(15,113)
Amortization of prior service cost (credit)	55	19
Amortization of actuarial losses (gains)	2,934	4,119
Net periodic benefit credit	(5,808)	(668)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	23	22
Interest cost	142	185
Amortization of prior service cost (credit)	(483)	(582)
Amortization of actuarial losses (gains)	(70)	(167)
Net periodic benefit credit	$ (388)	$ (542)