Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,258	$ 3,721
Interest cost	7,049	6,309
Expected return on plan assets	(15,113)	(17,123)
Amortization of prior service cost (credit)	19	35
Amortization of actuarial losses (gains)	4,119	1,786
Net periodic benefit credit	(668)	(5,272)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	22	31
Interest cost	185	163
Amortization of prior service cost (credit)	(582)	(649)
Amortization of actuarial losses (gains)	(167)	(119)
Net periodic benefit credit	$ (542)	$ (574)