Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,254	$ 3,710	$ 6,512	$ 7,431
Interest cost	7,043	6,296	14,092	12,605
Expected return on plan assets	(15,112)	(17,122)	(30,225)	(34,245)
Amortization of prior service cost (credit)	21	34	40	69
Amortization of actuarial losses (gains)	4,121	1,787	8,240	3,573
Net periodic benefit credit	(673)	(5,295)	(1,341)	(10,567)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	22	32	44	63
Interest cost	184	162	369	325
Amortization of prior service cost (credit)	(581)	(650)	(1,163)	(1,299)
Amortization of actuarial losses (gains)	(166)	(119)	(333)	(238)
Net periodic benefit credit	$ (541)	$ (575)	$ (1,083)	$ (1,149)