Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,721	$ 3,168
Interest cost	6,309	6,270
Expected return on plan assets	(17,123)	(15,713)
Amortization of prior service cost (credit)	35	80
Amortization of actuarial losses (gains)	1,786	1,853
Net periodic benefit credit	(5,272)	(4,342)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	31	36
Interest cost	163	176
Amortization of prior service cost (credit)	(649)	(853)
Amortization of actuarial losses (gains)	(119)	(139)
Net periodic benefit credit	$ (574)	$ (780)